Well; there are some things I usually notice when they say my signature is wrong ---
 (1) I have never got that crap from a male cashier

 (2) wen you are dressed in casual wear

 (3) wen you are removing amounts and you dont look like you can afford that amount

So the challenge I usually pose them is "I have an ID; two, the ID has my signature; three, you have a photo in my account - am sure there is a way you can pull it from the database"