Checking gender of fish can sometimes be difficult. But clownfish are a little easier. They are hermaphrodites, that is they change sex depending on how many males and females are in their group. The largest clownfish will become the female and the 2nd largest will be the male. These 2 will be the breeders of the group. The others remain non-breeding until one of them dies. Then the next largest clown fish will take that one's place. Since gender varies with different species of fish, it's best to research each variety to try and find what the differences are between males and females.