Which of the following is an ineffective way to gauge a customer's intoxication level?

Providing additional drinks without observation is indeed an ineffective way to gauge a customer's intoxication level. This approach does not allow for any assessment of how the customer is responding to alcohol consumed. By serving more drinks without being aware of the customer's behavior or previous alcohol intake, the server risks contributing to potential overconsumption. Observing a customer's interactions, body language, and even asking about their drinking habits are all proactive strategies that help assess their level of intoxication. These approaches offer valuable insights into how alcohol might be affecting the individual, whereas the option of serving drinks without monitoring them essentially neglects the responsibility of ensuring safe alcohol service.