I find most used guns fall into one of two categories: Toys that someone bought and got bored with, which are sold when the wife gets pregnant, or guns that have been arouns a LOT of blocks a LOT of times. There are of couse, guns that serious owners and collectors rotate out of their collections for practical reasons, but I think that these don't usually wind up in pawn shops. (I'm sure a few pawn shop operators will disagree with that.)
I've had good and bad experiences with both new and used guns. Keep in mind, modern handguns are meant to be very rugged and durable, pretty much to a ridiculous degree of redunduncy. (Especially those designed for police and military bids.) You have to work pretty hard to make most handguns really fail to a point that a good cleaning won't cure.
The guy you're buying it from should be reasonable enough to let you look at it. There is a lot you can determine by kicking the tires in front of him, but any gunsmith would be happy to look it over. Massad Ayoob wrote a great piece about buying a used gun in the most recent edition of "The Complete Book of combat Handgunnery", which I tell everyone to read anyway. It includes methods of testing firing pin strike, return to battery, etc, but a real look by a gunsmith covers these things.
A CZ or a Glock makes my VERY short list of guns I will buy used with very little inspection. The previous owner might have messed it up, but he would have to try REALLY hard. I would buy say, a 1911, but I would do it with the intent of a complete detail strip, inspection, and replacement of a few parts anyway, letting me REALLY see what shape it's in.