Use App Scanner. It analyzes your .app file for private API method usage.
It will catch methods that have been named the same as a private API method, even if it has it's own implementation. Also, it'll catch @selectors inside methods (just like the official iOS automated checker).
App Scanner doesn't catch the case when performSelector: is called with a selector that is a private method. However, Apple's tools do catch that case.
Another Alternative APP Kit, though I haven't used it. Give it a try.
Check out this Q&A, if you haven't. How does Apple know you are using private API?. Looks Helpful.