Question: Is it possible to detect unused methods in a source tree using FindBugs? I see some posts on SO where users are claiming to do that, some others asking how to do this in FB and others where they claim FB cannot do this. 

Does anyone know for sure how this is done? I am only interested in methods that are not explicitly called from elsewhere, I don't care about reflection.

Answer: 
I suppose it would be quite possible for Findbugs to report on public methods which are not used the same way it reports on privates (either that or I'm thinking of a compiler flag :-). 

The real question is why would you want too? If you are writing a program which is closed and will never be extended, then locating unused methods gives you an opportunity to remove them. But if you are writing an API you cannot predict who will need those methods so there is not much point in reporting on them.
Maybe crap4j is what you need. It removes all code that is not reached by unit tests. This is of course the hard way to minimize your app.