Which is the most reliable basis for a server's decision that a customer is intoxicated?

The most reliable basis for deciding a customer is intoxicated is the server's own observation of physical and behavioral signs of impairment — slurred speech, poor coordination, bloodshot eyes, impaired judgment, and behavioral changes. A customer's self-assessment ('I'm fine') is unreliable, since impaired judgment is itself a symptom of intoxication. Servers are trained to trust observable signs over a customer's reassurances or irrelevant factors. Basing service decisions on careful observation is the professional, defensible approach to identifying intoxication.