To tell male and female crayfish apart, turn them upside down; the male has two tiny extra leg-like protrusions behind the last set of legs. It is safe to have the crayfish out of water for a short time to determine this.
Hold the crayfish gently right behind the claws with your thumb and forefinger. The crayfish are well protected externally and should not be hurt by pinching them in this way. Turn the crayfish over. Look down below the last set of legs. If there are two tiny protuberances that look like little extra legs, the crayfish is male. If these are not present, it is female.