How do you determine the gender of a fish?

How do you determine the gender of a fish?

You give the fish some food. If HE ate the food, it is a male fish. If SHE ate the food, it is a female fish.

EDIT: Explanation: "If HE/SHE ate the food" (HE is specified as a male fish, just like SHE is specified as a female fish)

Sorry for bringing retarded Croatian jokes to /r/Jokes