My brother is a drummer so I use to be able to mess around on his kit once in a while. He also took lessons and studied with a few other local pros in our area.
From what I know of it, drummers usually start off practicing what's known to them as rudiments, which is a series of alternating stick patters. These patters are usually what makes up certain rhythms. You can usually tell if he knows them or not by listening to how he does fills. If everything he does is nothing more than 1/8 & 1/16 note rolls more than likely he has never studied.
Just say to him, "hey man I dig your double stroke roll". if he gives you that deer in the headlights look, just move on.
