This is what my good friend who owns a pawn shop does:
If somebody says that he's buying a gun as a gift, he just asks a few questions. Just casual stuff, like "is it a special occasion?" or "man, you're a real pal! How long have you guys been friends?" Simple questions. If he gets that "feeling", he won't complete the sale. Given that he runs a pawn shop, now and then the caliber of his customers is...well...sometimes not so good. He takes his obligation to keep guns on the side of the law seriously - after all, from a purely selfish standard, who wants to take the chance of having to look down the business end of your own merchandise?
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae