There are two main ways to tell the difference between a male and female crab. The first is to inspect the claws of the crab. Males usually have one claw that is longer than the other. If you are looking at blue crabs, you can also distinguish by color. The females will have claws with red tips while the males will have claws with blue tips. The second way is to look the crab's abdomen. A male crab will have a more narrow, slightly triangular shape in its middle. A female crab's abdomen will be more broad and rounder than a male's.