Your turtle is way too young to sex yet Dan, almost impossible to tell untill they are much bigger. later when its a little bigger sex will be more apparent, males have slightly concave plastrons (slight dip in lower shell) to make mounting the female easier. Males generally have longer tails than females, having a few makes comparisons easier. The male is generally smaller carapace length than females and his cloaca is closer the end of the tail.
Males, if you look at the plastron where the tail emerges, usually has a more rounded 'u' shaped notch, females notch being more 'v' shaped. Id try again when it reaches about 10 cm. try to get pics of know male and female so you can compare.
