How do you tell the difference between male yellow-bellied sliders and female yellow-bellied sliders?

Length of Nails

Males have long claws (aka toenails) on their front and back feet. Females have short nails. I dont know about the "length of the tail" answer, I have never heard that before, and I have two yellow bellied sliders that I raised from hatchlings.

by how long the tail is maleshort tail=female

long tail =male

Males of most turtle species will have a shallow (sometimes deep esp. with age) concavity to their plastrons (the bottom shell). This is for mating because the male climbs on the female's back.