i work at teh gas station, so i'm handeling credit cards all the time, i watch for signatures, i watch:
-the speed of the signature
-the likeness of the signatures
-and the reactions of teh person
-typicaly if they tryt to snach it back fast i make it very clear that i'm watching there signature
-but i've never had to ask for ID on a credit card, just smokes and lottery
next time someone asked you for id, ask them for there ID