Final answer:
When off duty at a convenience store and the cashier mentions receiving a counterfeit bill, you should report the incident to the store manager.
Step-by-step explanation:
If you are off duty and in a convenience store when the cashier mentions that she believes she received a counterfeit twenty-dollar bill, the most appropriate response would be to report the incident to the store manager. Since you are not on duty and the person does not know that you are a police officer, it is not necessary to inform her about your profession. Additionally, confronting the cashier or accusing her of accepting counterfeit money without any evidence would not be appropriate in this situation.