It's delusional to think you can know by appearance. I personally know some rough-looking guys who are upstanding citizens, and I know at least one person who presents himself well but is prohibited from owning firearms....
Doesn't appear to be a gangsta, thug, doper or tweaker, check.
...
I don't often sell, but I'd still ask for DL and LTCH if I don't know the buyer. The legal minimum of not knowingly selling to a prohibited person is ridiculous - that's a loophole you can drive a truck through. As a responsible gun owner, I feel an obligation to do what I can to prevent a gun from falling into the wrong hands. It's sad that so many people don't seem to care.
And don't give me the line that "criminals don't follow laws anyway" - that's a cop out - why bother having laws in the first place?