The safest response to any genuine doubt about a customer's age or sobriety is to:

The safest response to any genuine doubt about a customer's age or sobriety is simply not to serve until the doubt is resolved — by verifying a valid ID, consulting a manager, or determining the customer is not intoxicated. This 'when in doubt, don't serve' principle runs through all of responsible beverage service, because the consequences of an illegal sale (to a minor or an intoxicated person) are far more serious than a brief delay or a declined sale. Erring on the side of caution consistently protects the server, the establishment, and the public.