What should you do if you have checked a guest’s ID and are still not sure if she is old enough to drink?

Requesting a second form of identification can help confirm the guest's age and provide additional verification. The second ID should also be an acceptable form of identification, such as a government-issued photo ID or a passport.
Relying on another server's opinion may not be reliable, as it is ultimately your responsibility to ensure compliance with age restrictions. Asking a question only a person of their age would know or asking the guest's friend to verify her age may not provide sufficient evidence and can be subjective.