Final answer:
Banks typically verify a customer's signature by comparing it to the signature card the customer filled in upon opening the account. This signature serves as a reference for all subsequent transactions requiring signature authentication.
Step-by-step explanation:
To address the question of how does a bank verify the signature of its customers, most commonly, banks rely on the signature card that the customer fills out when they open an account. This card serves as a reference for the bank when verifying the authenticity of the customer's signature on checks and other documents. The other methods listed, such as using a substitute check, documents like passports or ID cards, or a pin number, are not typically used for signature verification in banking transactions. Banks maintain these signature cards on file and whenever a transaction such as a check is presented, the signature on the transaction document is compared to the one on the signature card to ensure they match.