The size of the claws makes it very easy to tell the difference between male and female fiddler crabs. A male fiddler crab has one claw that is very large compared both to its other claw and the claws of the females. The females have two small claws.
The photo to the right gives a side by side comparison. Clicking on the photo will allow you to view larger photos of both the male and female for comparison (click on the "next" button under the image that pops up).