The last few times I've paid in a restaurant using a card in the US or Canada, they've not been able to check the signature.

I ask for the bill and they come over with it.
I give them my card and they go print up the CC receipt.
They give me the card back and the two copies of the receipt.
I sign and give them back their copy but they don't get my card back.

Unless the person has amazing memory, they don't have anything to compare to.