Rust is a big giveaway. Dings and scratches to excess especially on the muzzle is a no-no. Cracked parts. Dirty bore. Bores should be bright and shiny. If the person selling a used gun isnt willing or able to run a couple patches down the bore skip it, the dirt is likely hiding rust.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent