Typically you just clean the general area of the leak and watch for it to leak. However now they sell special kits that have dyes the you can add into the engine to help pinpoint the leak. They come with glasses and a UV light to help locate the leak.
Typically you just clean the whole engine and carefully inspect all around it to see where it is leaking. They do now sell kits that include special uv dyes that can be used to help find the leaks. You can try one if you can see where it is leaking.