Aside from an arm wrestle, the quickest way to separate the men from the boys is to examine their facial hair.
It might seem a bit prehistoric but the ability to grow and maintain a beard is still viewed as a sign of true masculinity. In case you don't subscribe to our deification of facial hair, we have an infographic to teach you a damn lesson.
Read it and weep (if you're unable to grow a beard).
(Image: Rex Features)