If you are not certain that an ID is valid, what should you do?

If you are not certain that an ID is valid, the appropriate course of action is to ask the guest for a second valid ID. Requesting an additional form of identification can help establish the guest's identity and provide further verification. It is important to handle such situations with professionalism, respect, and adherence to applicable laws and regulations.
Calling the local police should only be done if you suspect illegal activity or if there is an immediate threat to safety. Questioning the guests until they give up and leave is not an appropriate or effective approach. Serving a weaker version of the drink ordered is not a recommended solution either, as it does not address the issue of verifying the guest's age or identity.