A visleak will show in your compile log. If there were no problems during compile, you had no leaks. a leak is "discovered" when a leafnode is created with one of it's portal's exposed to "nothingness", or the void. The closest entity to the void will "leak" and be used as a reference point to direct you to the offending hole in your map.
All you need to do is block line of sight to these area's that can be seen through your skybox. This is often a difficult problem to overcome when incorporating height changes in your map. But this is not a leak.
That guy doesn't know what he's talking about. There can't be a "ton" of visleaks, there will be one regardless of amount of holes, and they will leak one by one, per compile, untill the author seals them all. If there was a leak that broke vvis from running during compile it would cause the whole map to be rendered at once, so he may have jumped to conclusion in this respect, by seeing another section of the map through the skybox; but the skybox has strange properties i couldn't begin to define, one of them is allowing textures to be rendered directly beyond it's bounds. The only way to tell if there was a leak whilst ingame is having cheats on, developer mode on, and mat_visleaf 3 set; that will show you your leafnodes. If there are no leafnodes, there was a leak, vvis did not run, and your whole map is rendered at once.
