Goldfish are pretty easy to tell if they are a male or female by looking at their body shape or character. Although some may never be able to tell because their goldfish hasn't lived long enough to mature. Male goldfish are more slim and longer. Females have more of a rounder exterior. Males can just be defined on how they swim. They are always swimming under the female or nudging her, as if he wants to play. So next time you go by your fish tank, look at your goldfish and you'll might want to change it's name.