Generally I stay out of gun stores. But when I am in one and see something interesting to me, it's usually the case that I know far more about the thing than the guy selling it. (BECAUSE it's something that interests me) I will often ask a middle of the road question I already know the answer to, not something stupidly obvious, but not something only a super geek collector would know. How they answer will determine whether I keep looking at it or not. If they know the answer, or admit they don't know the answer, I stay interested. If they started double talking or flat at making shit up, I walk away. Sometimes I'll leave them flat, other times I'll tell them all the reasons why what they said was a lie, as I'm walking out.
I had a family member and friends who sold guns part time. I told them all, "The best thing I can hear a gun store clerk say is "I don't know", because at least I know they're being honest."