How can you tell if comet goldfish is a male or female?

A male has more of a V shaped tail. A female has more of a U or a rounded V and

First if your comets are not at least 2 years of age, they won't breed.

Males vent openings are concave or sunken in while the females vent opening is convex or poking out. The males have firmer dorsal fins and are more streamline than the females. The females are heavier in the abdomen area than the males.

Breeding tubers won't appear on the males gillcovers or fins until spawn. That is to say you don't have two females or two males.

They will breed once they are old enough and if they are male and female