How do you know if an alligator is a boy or girl?

An alligator can be identified by it's physical aspects or appearance.

Crocodiles and alligators do not have any distinguishing features externally to tell male from female, apart from size. Males grow considerably larger than females.

Male American alligators for example can reach sizes of up to 20 feet, whereas females hardly ever grow over 12 feet in length. Another way to determine the gender is to insert a finger into the cloaca or vent in order to determine it's gender. This should only be done by professionals.