Bill,
Main thing is, you have to know the make/model you're looking at well enough to be able to determine what's normal and what's not.
The very first thing I look for in a used gun I'm considering for purchase is evidence of abuse in the form of 'kitchen table gunsmithing.' Some folks just cannot resist taking things apart that don't need to be taken apart, and those folks are generally pretty ham-fisted in doing so. Evidence includes chewed-up screw slots, flared pin holes, parts and pieces that just don't fit right, looseness and play where there shouldn't be any, etc. That sort of thing is a flat out turnoff to me. Sometimes guns break and need to be fixed, but someone who knows what they're doing won't leave elephant tracks behind them.
Generally a certain amount of simple neglect and honest wear doesn't bother me too much in a used gun, if the price is right. Shallow external rust and dirt clean up pretty easily. Dings in furniture are to be expected. Even accumulated gunk, dried-out oil, etc. inside an action will clean up pretty easily. I once pulled the trigger plate out of an old Wingmaster I was looking at and had enough weed seed fluff fall out to line a couple of bird's nests. Sorta shocked the owner of the gun shop, who had given me permission to push out the pins (when 870 shopping I always carry a long wooden golf tee in my pocket to push pins with) so I could examine the trigger plate assembly for evidence of abuse or repairs. Gross negligence, deep rust, internal rust etc, are bad signs.
I know the owners of most of the FFL shops I deal with pretty well and they don't mind letting me look a little closer at used stuff. But I always ask first and stay out of the way while I look guns over. I've saved several of them from trouble with bring-backs by unhappy customers when I found problems with used guns the staff didn't catch in purchasing the item, and they appreciate that. In a place where I don't know the owner/manager I'm a lot more careful.
hth,
lpl