A male platy has an anal fin that is more elongated and the female's anal fin is more fan-shaped. This anal fin comparison also works on guppies, mollies and swordtails.
By being able to determine the sex of platys, it will be easy to breed the fish in a home aquarium; alternatively, if an aquarist chooses not to breed, it is easy to buy fish of the same sex. Fish keepers should avoid buying a tank full of male fish as they can become aggressive with one another. To avoid the hassle of raising fry, filling the tank with females is the best option.