If you are actively looking for unused code, use something Jake223 suggested.
But, if you just stumble upon some code that seems to be unused, you can just do a search for the method's name in your codebase. If it's a private method, your job is easy, search only the current class. If it's a public method, search everything. If it's a whole class, search for it's name in all your code.
No hit? Perfect. Delete the code and run tests. You do have tests, don't you? Than commit your changes to your document version system. You use one, don't you? That way if you discover, ever, that you need it back, you just revert a change.
Deleting code should be something you do every day. Maybe you refactor some code and delete the old one. Or find old, unused code and delete it. And even if you don't have backups, how hard it is to rewrite a function or two? And the second time you will certainly write them better than before.