Ok...I just talked to one of my Army colleagues and he says that the guy should have a DD-214 if he's out or, if he's Army, an ERB (Enlisted Record Brief, will show his deployments) if he's still in. If he doesn't have one of those two things, he's very likely a liar.
The catch, of course, is working up the nerve to confront a friend.

I wouldn't want to call-out a vet either only to find out that he's legit. Though, for me, lying about military service is much worse than asking a vet to produce a DD-214 in order to help protect his integrity. I'd like to think that, if the pledge has served our country, then he would understand and wouldn't want anyone making false claims either.
.....Kelly