Grasp the crayfish firmly by its shell, behind the pincers. Flip it over, so that you can look around and behind its legs. The first anatomical features that you should look for are the modified swimmerets, present on males but not on females. The first two male swimmerets are enlarged, and are used to transfer sperm to the female. Female crayfish lack similarly enlarged swimmerets.
Look at the base of the third pair of swimmerets. On the female, you will see certain openings behind the swimmerets, called oviducts. Oviducts look like small, dark holes near the base of the swimmerets.
Look for openings behind the fourth walking legs, situated nearest the tail of the animal. Openings here are generally openings for the vas deferens, on the male crayfish.