CaBSP has become very good in reliably detecting leaks, and when CaBSP aborts compiling and complains about leaks, it makes pretty clear in its error message what's going on.

However, if this is your first leak, I'd much recommend to read the great explanation in the Hammer online help in the Troubleshooting section: “How do I fix leaks in my level?”. It also teaches you what leaks are and how you can prevent them. Here are a few additional remarks that might be helpful:

1. Run Check for Problems from the CaWE Map menu.
2. Do never assume that translucent or detail entities seal your world. Sky brushes do.
3. Carefully look for misaligned brushes. In my experience, the “Carve” tool, rotation and certain other operations easily cause rounding errors which in turn cause leaks.
4. The best you can do is to prevent leaks from the very beginning by using the grid and avoiding all tools that potentially introduce rounding errors. As a gross guideline, all operations that operate on the grid and that are guaranteed to produce vertices on the grid are usually safe, such as resizing rectangular blocks, shearing, vertex manipulation (although that can introduce other errors, which can be identified with the Check for Problems item from the Map menu), mirroring, and clipping and carving, when the resulting vertices are on-grid. The following operations are usually less safe: Arbitrary rotation, and clipping and carving, when the result has vertices that are off-grid.
   For less-safe operations it is usually better to try to mimic the same effect with the safer operations. For example, when you want an arched door in a wall, creating and placing the wall brushes manually is much preferred over carving an irregular hole into the wall!

Finally, let me mention a very rare situation in which CaBSP reports no leak, but a leak is present: The symptoms of such unreported leaks are a blank screen when you load the world in Ca3DE, only environmental background is displayed, or you are falling through the floor and everything disappears after a few seconds. Such leaks occur sometimes when some old BSP decompile tools that are still available on the internet, produce world geometry that is so degenerate that not even CaBSPs rounding error control and validity tests can help it. It is possible to remedy such cases automatically, but I will write a description for the process only should somebody actually ever experience this problem.