How do I tell what gender my turtle is?

A friend of mine has a little pet turtle, and wants to get another one. His question was, what are the odds of reproduction if he just randomly gets another one (its 50-50, i know that one!) but we can increase the odds by knowing what gender they are. Any ideas? Or does it depend on the specific species?

if the turtle has a triangle on its stomach between it legs then its a girl if there is no triangle then its a boy

I am assuming it is a Painted turtle or Red-ear slider. The things to look for are:Tail: Long, skinny tail- male. Short wide tail- female.Belly: Incurved- male. Flat or out-curved- femaleFront claws: Long- male. Like the rear claws- female.Younger turtles are harder to know for sure.Turtle reproduction is not easy. It generally needs big tanks, lots of room, a small 'herd' of turtles (rarely will 1 of each reproduce. It is better to have at least 2 females.) You'll need to hibernate the turtles and offer a good nesting site. But, you'll have time. Turtle's don't hit sexual maturity until about 5-7 years old. On the other hand, if they were badly cared for in the meantime, they won't mature right.