Answer:
Consult your state's Uniform Commercial Code 4-302. Also note that under UCC 4-401 the responsibility for failure to return a forged or counterfeit check timely falls to the depository institution for honoring a check that was not properly payable. The customer would only be liable if the depository institution felt it could prove that negligence on behalf of the customer contributed to the loss, or the notice of counterfeit item was not timely under UCC 4-406.
If you are unsure of the application of the laws in your state, you should consult with legal counsel as my response should not be considered legal advice.