how to check if a gun is stolen or been involved in crime?

im allowed to sell guns that ive bought, used for a while and then sold?

and idk damirs friend is a peace keeper or some shit and he said something wasnt right with it, idk exactly what the issue was but I just returned his money and that was that.

not gonna go through all the trouble to call the ATF lol, I just wanna know how most people say on wantad buy guns or if they just take the sellers word for it not being hot.