The sex depends on the kind of turtle but there are some basic rules;

1. Some turtles can't be judged until they are two years old.

2. Many male turtles have in-curved bellies and female turtles have flat or out-curved bellies.

3. Many male turtles have longer, thinner tails and vent near the middle. And female turtles have short wide tails. The vent in females are near the base of the tail.

In some species of turtles you have to see other things. For more information, visit the link below.
Sex of Turtles