You can probably trust your instincts... if it's a $10,000 race bike and the seller is a beer guzzling slob, it's probably not his. If he's 5'6" and the bike is a 62 cm frame, it's probably not his. If he's "selling it for a friend" it's probably not his.

And frankly, I don't have proof of purchase for any of my bikes although I might have a really wrinkled shifter manual laying around, so I'm not sure that's an infallible sign either.