If its a male, when he gets to be appx. 8 to 10" from front to back he will show you his "stuff." On females its a little harder and you have to wait a little longer. Males have a longer fatter tail. Females have a short stubby tail, but you usually can't see a difference until they get to be around 8 to 10".
Sometimes, the plastron scutes closest to the tail give an indication; from what I've gathered, if the scutes fan out {much like the tail fin of a fish} then it's male, and if the scutes curve around {much like the letter "u"} then it's a female. Of course, this is not "scientific", but can give an idea without having to wait years.
Somewhere on here, you will find mention made of this, along with some pic's to show the difference; just browse around and I'll bet you come across it.