No, you can't see the signatures, so they don't help you locate fraudulent transactions. What they do is provide a defense for the merchant against you. If you

No, you can't see the signatures, so they don't help you locate fraudulent transactions. What they do is provide a defense for the merchant against you. If you try to say you didn't authorize the charge, then the merchant can produce the signature as proof that you did. The signature is never evidence that you weren't there. Even if it is someone else's name, that doesn't prove anything, so it never benefits you.