Final answer:
If a cashier mentions receiving a counterfeit bill, advise them to contact the police.
Step-by-step explanation:
If you are off duty and a cashier mentions that they believe they received a counterfeit twenty-dollar bill, the best course of action would be to advise the cashier to contact the police. As a civilian, you don't have the authority to take any formal action or investigate the matter yourself. By advising the cashier to contact the police, you are ensuring that the appropriate authorities are notified and can handle the situation.