I'm wondering how wizard identification works. In my story, a bartender only serves wizards of age, but how does he know? Do they carry I.D. around like we do? Or is there a magical test that the bartender can easily do to check their age?

If there is a test, how does it work? Does it give age? Name? Address? Can it be tricked (like Fred and George tried to trick the Goblet)? Does the person have to willingly submit to the test or can anyone walk up and find who you are?

I have lots of questions! Thanks so much for any help!
Kristen