Unlike other species of turtles, determining the sex of a painted turtle is fairly easy. It is not possible, however, to determine the sex until the turtle has reached maturity.
Check the turtle's tail
Turn the turtle upside down, and look at the tail. The male turtle has a significantly longer and wider tail than the female.
Look at the turtle's cloacal opening
Keep the turtle upside down, and look at the cloacal opening. The male's cloacal opening appears past the end of the carapace, while the female's is near the base of the tail.
Inspect the turtle's claws
Look at the turtle's front claws. Male painted turtles' claws are significantly longer than those of females'.