how do you determine the sex of an red eared slider (turtle) ?

Answer 1

Its a bit difficult while they're young and still growing to know for sure but its the length of their tail, whether its thinner (girl) or fatter (boy) and where their excrement hole is located on the tail - if its further down the tail its a boy and if its closer to the body is a girl.

Answer 2

The length of the tail.
Longer = male
Shorter = female