Determining Minecraft mod abilities/effects

Using a tool that is able to search inside of zip files, you could search your mod folder for worldgen-related terms (eg. biome/generator/...). This is not certain to find every relevant mod, although it might help you to narrow it down.

If you are really interested in the world, you could decompile the source files and then search them for worldgen event registrations: every mod that tries to modify world generation has to 'sign up' to get involved in that process, and this 'signing up' call should be very easy to detect (you can look up the specific event names in the minecraft forge forums).