A male crawfish will start to develop its signature bright red pigmentation on their claws during Juvenile stage. Whereas females, there is no colour on their claws.
Another way to tell the gender is by looking at the underside of their bodies.
For female crawfish, there will be 2 holes and for male crawfish, there will be 2 pultruding reproductive organs as seen in the picture below.
Therefore, when adult crawfish reaches maturity level, the male will flip the female over to fertilise her.