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...