It's picture time! I'll work through each stage. All of the blue dots are acceptable offensive sentry locations. All of the red dots are acceptable defensive sentry locations. The thing about sentry locations is that if there's a limited number of good spots the attackers will always look to those same spots. This is true on Dustbowl until you reach the second half of the 3rd stage, where it opens up and the good locations increase quite a bit. So it's generally much better to design areas instead of specific sentry spots, if that makes sense.
Things that make a good defensive sentry location:
1. Limited long sight lines from the attacker's side of the map.
2. Nearby metal (medium or large).
3. View of heavily traveled areas, especially choke points.
4. An escape path (if your sentry is about to go down).
5. Higher ground.
6. Lack of lower ground (ie spam traps, which catch nades and funnel them).
7. Protected corners that you can sit behind while wrenching your sentry.
8. Reverse views of cart paths (build pointing in the same direction as the cart movement, but from a hidden spot so the sentry hits the attackers from behind).
9. Overhead obstacles to prevent stickies from coming over walls, buildings, etc.
Roofs generally make excellent sentry locations if they're not in really long sight lines. The reason is that they generally have limited access to them, but provide an infinite number of escape routes for the engineer. They're higher ground and a teleporter can help bring many teammates to these good locations. For an example play on pl_happycow sometime.
Also, a good rule of thumb is to reduce areas where a sentry can't shoot across from one side to the other. This increases the number of good natural spots by quite a bit.
The letters are for areas that could be converted to good sentry locations without a huge amount of work.
A: With a hoodoo at the large dot to reduce sight lines and an escape tunnel behind, this is probably the best forward spot for a sentry. The carts should be moved/removed for a more open line of sight in that direction. This isn't overpowered (OP) because of the line of sight from the far path and the fact that it's a nade trap and can be cornered by a pyro easily. By the way, the red dot above is my favorite sentry position. I put a level 2 there and nobody notices until it gets a handful of kills.
B: This would require a balcony on the rear half of the building, and a stairway leading up. It needs to overlook the cart path while being protected from the blue spawn. It's not OP because demomen can attack from below easily.
C: This spot is mostly useless right now because it can get hit so easily from the far ledge without being able to shoot that far back. To change this, you'd have to lower the height of the windows considerably and/or increase the depth of the room so a sentry would be more protected. Another option would be turning this into a 2 story structure with a balcony, assuming the balcony is within sentry range of the far ledge. Also, demomen on the nearby ledge can too easily bounce nades into that room. If the side door were turned into a window or closed completely it would help.
D : This is a fun spot to build, but kind of useless right now. It takes too long to get up there during setup since you need to build a dispenser. Then once you're up there you get no protection from the side facing the blue spawn. Increasing the height of the building with the white roof would help a lot, as would adding a stairway to the rear of the building. It's not OP since demomen can hug the building to sticky it, spies could get up with a stairway, and all classes can hit it from the buildings without taking much damage at that distance.
E: This would be an awesome spot. Make the upper level into a balcony that can overlook the path below from a couple of angles. Slightly OP, but people can just avoid it until it's taken out.
F: This would require that you remove the lower path. It seems people prefer the upper mine path for setting up forward bases/ubers anyway. In the next room over you could add a balcony in that corner and a lip around the wall so engineers can get over there. This wouldn't be OP since people could easily take it out by using the upper route, but it's an easy extra option for sentries.
G: Make this roof buildable and add a few boxes to the rear platform so people can access it. Requires plenty of planning from the engineer since he has to dispenser up the rock and then jump across, but it would be a really fun spot. Not OP since soldiers can hit from the far upper path and demomen can hit from the doorway below. Would be awesomely fun.
H: The problem with this spot is that soldiers and demomen can take it out from the upper 1 way door without any chance of being killed themselves. To remedy this you'd have to change up that side platform a bit by making it 2 levels. This would give the sentry a clear view of the door. Narrowing that entire platform would have the same effect, but I like the look of it wide like that.
Nearby I've X'd the little wall that doesn't seem to have a purpose other than robbing me of a decent sentry spot. 
I: By making full-height windows up here, you give a sentry a clear view of the entire spawn area. It'd get taken out pretty quick, but it's almost required since there's no good sentry spots up here right now.
J: Adding a small building/platform/ledge here would be excellent. A ramp or stairway should come up from the back. It's a good spot since you get a clear view of the cart as it comes around the corner as well as the shortcut door and upper ledge on the other side. It's not OP since attackers can hit it from long range through the 1-way door. It would make the stage much harder for the attackers, which I think is necessary.
K: The problem with this building now is that the large doorway and long sight lines makes it impossible to build here. Anything in here gets taken out really fast. To remedy this the doorway should be moved halfway down the building so that corner is completely protected. Windows should be left open on the rest of the building so a sentry in here can hit the path. A little OP, but demomen and ubers can still take it out pretty easily, and it would make this last area harder to attack overall.
Hope this helps.
I don't really think changes are necessary for hoodoo, but perhaps this will give you insight for your next project. 
Things that make a good defensive sentry location:
1. Limited long sight lines from the attacker's side of the map.
2. Nearby metal (medium or large).
3. View of heavily traveled areas, especially choke points.
4. An escape path (if your sentry is about to go down).
5. Higher ground.
6. Lack of lower ground (ie spam traps, which catch nades and funnel them).
7. Protected corners that you can sit behind while wrenching your sentry.
8. Reverse views of cart paths (build pointing in the same direction as the cart movement, but from a hidden spot so the sentry hits the attackers from behind).
9. Overhead obstacles to prevent stickies from coming over walls, buildings, etc.
Roofs generally make excellent sentry locations if they're not in really long sight lines. The reason is that they generally have limited access to them, but provide an infinite number of escape routes for the engineer. They're higher ground and a teleporter can help bring many teammates to these good locations. For an example play on pl_happycow sometime.
Also, a good rule of thumb is to reduce areas where a sentry can't shoot across from one side to the other. This increases the number of good natural spots by quite a bit.






The letters are for areas that could be converted to good sentry locations without a huge amount of work.
A: With a hoodoo at the large dot to reduce sight lines and an escape tunnel behind, this is probably the best forward spot for a sentry. The carts should be moved/removed for a more open line of sight in that direction. This isn't overpowered (OP) because of the line of sight from the far path and the fact that it's a nade trap and can be cornered by a pyro easily. By the way, the red dot above is my favorite sentry position. I put a level 2 there and nobody notices until it gets a handful of kills.
B: This would require a balcony on the rear half of the building, and a stairway leading up. It needs to overlook the cart path while being protected from the blue spawn. It's not OP because demomen can attack from below easily.
C: This spot is mostly useless right now because it can get hit so easily from the far ledge without being able to shoot that far back. To change this, you'd have to lower the height of the windows considerably and/or increase the depth of the room so a sentry would be more protected. Another option would be turning this into a 2 story structure with a balcony, assuming the balcony is within sentry range of the far ledge. Also, demomen on the nearby ledge can too easily bounce nades into that room. If the side door were turned into a window or closed completely it would help.
D : This is a fun spot to build, but kind of useless right now. It takes too long to get up there during setup since you need to build a dispenser. Then once you're up there you get no protection from the side facing the blue spawn. Increasing the height of the building with the white roof would help a lot, as would adding a stairway to the rear of the building. It's not OP since demomen can hug the building to sticky it, spies could get up with a stairway, and all classes can hit it from the buildings without taking much damage at that distance.
E: This would be an awesome spot. Make the upper level into a balcony that can overlook the path below from a couple of angles. Slightly OP, but people can just avoid it until it's taken out.
F: This would require that you remove the lower path. It seems people prefer the upper mine path for setting up forward bases/ubers anyway. In the next room over you could add a balcony in that corner and a lip around the wall so engineers can get over there. This wouldn't be OP since people could easily take it out by using the upper route, but it's an easy extra option for sentries.
G: Make this roof buildable and add a few boxes to the rear platform so people can access it. Requires plenty of planning from the engineer since he has to dispenser up the rock and then jump across, but it would be a really fun spot. Not OP since soldiers can hit from the far upper path and demomen can hit from the doorway below. Would be awesomely fun.
H: The problem with this spot is that soldiers and demomen can take it out from the upper 1 way door without any chance of being killed themselves. To remedy this you'd have to change up that side platform a bit by making it 2 levels. This would give the sentry a clear view of the door. Narrowing that entire platform would have the same effect, but I like the look of it wide like that.
I: By making full-height windows up here, you give a sentry a clear view of the entire spawn area. It'd get taken out pretty quick, but it's almost required since there's no good sentry spots up here right now.
J: Adding a small building/platform/ledge here would be excellent. A ramp or stairway should come up from the back. It's a good spot since you get a clear view of the cart as it comes around the corner as well as the shortcut door and upper ledge on the other side. It's not OP since attackers can hit it from long range through the 1-way door. It would make the stage much harder for the attackers, which I think is necessary.
K: The problem with this building now is that the large doorway and long sight lines makes it impossible to build here. Anything in here gets taken out really fast. To remedy this the doorway should be moved halfway down the building so that corner is completely protected. Windows should be left open on the rest of the building so a sentry in here can hit the path. A little OP, but demomen and ubers can still take it out pretty easily, and it would make this last area harder to attack overall.
Hope this helps.