The best way to tell if a crawfish is male or female is to open it up and look for the sexual organs, which are found in front of the heart. The male's gonads are white, and the female's are orange.
A kinder way to tell a male crawfish from a female is to find one during the breeding season, which is in autumn. Some species of crawfish are found in streams, while others are found in swamps and roadside ditches. To determine the sex of the animal, turn it upside down. If eggs are attached to the swimmerettes, it's a female. Swimmerettes, or pleopods, are leglike appendages on the underside of the abdomen of a crawfish. They help the animal gather food and to brood eggs and larvae.
A large female can produce hundreds of eggs. Fertilization occurs inside the body, then the eggs are released into the water. The female then uses her legs and abdomen to waft them to her swimmerettes.
The eggs hatch into larvae by spring and cling to the mother until they mature, which takes roughly three months.
Another method is to look at the legs. The males of some species have hooks on the third and fourth walking legs, which are found in front of the swimmerettes.