I guess depending on the subspecies you can expect them to be mature at around 11-12cm long? Not sure, maybe someone with experience with this animals will chime in.
What you need to look for is the cloaca. Enlarged means male. The difference between male and female cloacas is hard to miss. There are other signs like adult size, over-all roundness of the body....but the cloaca usually is more than enough to tell.