It's easy to tell the difference the males have thicker tails and longer nails. The females have short skinny tails there bigger than the males and have shorter nails that's how you tell the difference
In order to tell the difference from a male or a female turtle, look at the plastron( bottom shell of a turtle) and if the plastron is flat from the front of the turtle but has a curve at the end by the tale, it's a girl and if the plastron is flat at the front of the turtle and flat to the tail of the turtle,
If your turtles are very small then let them grow a bit. But if they are fully grown then it is very easy to find out their gender. You can see it by watching closely its shell from its back. At the back of shall you will find a growth, which can be like a bud. That is male turtle while female turtle has no extension at its back. Watch it carefully or use a torch if extension is very small.