How do you tell a land turtle if it is a boy or girl?

The shell of most turtles is known in 2 parts. A plastron (bottom part) and a carapace (upper or rounded shell). On the female the plastron is flat, where as the male has a shallow indent back by the tail (for mounting purposes). Also the female has a wider tail starting at the base and the male has a slimmer evenly shaped tail.