To elaborate a bit more;
When you create a brush using the brush tool, it starts out as a World brush. It can then be turned into a func_detail or other brush based entity by hitting the "To Entity" button on the right side of the interface.
Once it's an entity, it's useful for all kinds of things, but it is no longer capable of sealing your map. Neither are displacements.
The inside of your map must be 100% sealed using WORLD brushes. If you have a brush that is currently a func_detail or some other such, you can hit the "To World" button at right to turn it back.
As mentioned, the pointfile is the best way to check for leaks, but a very basic gut check you can do is to just use the 3d camera to fly outside the map. Fly around in the void looking at your map. If, from the outside, you can see ANYTHING on the inside that the players could see, that's going to be a problem. From the outside, all you should be able to see is skybox and the back side of world brushes.