You really can't miss the bulbous pedipalps, they're more than obvious. If you have to ask if it's a mature male... then it's not. That is, if you've seen more than a couple tarantulas that aren't MM's.
The pedipalps get thicker, shorter, and almost look diseased. The first time I ever had a MM, I legitimately thought that something was severely wrong with him. See the red area that almost looks like an injury? Those are it.