Here is a good place to start.
While some states have laws barring a cashier from writing down your ID information, the card networks -- Visa, MasterCard and the like -- have the most say in whether merchants can even ask for identification. The agreements merchants sign when they decide to accept cards from those networks include rules that govern card verification procedures.
On the other hand, Visa and MasterCard rules prohibit the acceptance of unsigned cards. If you present one, the merchant must ask you to sign the card and supply an ID. Visa guidelines specify that it must be an official government ID.
Merchants may ask a customer for identification, but in most situations, a merchant may not condition acceptance of a Visa or MasterCard credit card upon the customer presenting identification. In other words, you can refuse to provide identification, and the merchant still must accept your credit card.
From Mastercard's own policy:
Unsigned Cards
If a MasterCard Card is presented to a Merchant representative and the Card is not signed, the Merchant representative must:
Obtain an authorization from the Issuer,
Ask the Cardholder to provide identification (but not record the Cardholder identification information); and
Require the Cardholder to sign the Card.
The Merchant must not complete the Transaction if the Cardholder refuses to sign the Card.
Suspicious Cards
If a MasterCard Card is presented and the Merchant representative believes that there is a discrepancy in the signature, or if the last four digits of the PAN on the Card front do not match the four-digit truncated PAN on the signature panel or displayed on the POS Terminal, or if the photographic identification is uncertain, the Merchant must contact its Acquirer for instructions.
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