Is your turtle old enough to sex? It needs to be 3 or 4 years old usually, then you can tell by looking at the front claws which will be longer in a male than in a female.
The tail will also be longer and fatter in a male, and the vent will be nearer the tip, in a female the tail is quite short and the vent is roughly in line with the rear of the carapace.
It helps if you have a male and a female to compare the two!
If the turt is old enough and you can post some good clear pictures I may be able to tell you what it is.