Remember that secondary sexual characteristics in turtles are more dependant on size than on age, and larger turtles will require a larger size to show those than smaller turtles. For example, most red-eared slider males don't show secondary sexual characteristics until 4-4.5" in straight carapace length. For eastern painteds it seems to be about 3", and for redbellies about 5".
In all turtles and tortoises, the male has a longer, thicker tail, with the cloaca being located beyond the edge of the carapace. In some species the difference is so great that it is shocking, such as with mud turtles. If you don't find yourself going "Holy crap!" when you look at the tail of a mud turlte, it's a female.
For most US basking aquatics, males have longer front nails. They will NOT have a concave plastron, as there is no need for this in deep water.
For box turtles, there are a number of ways to tell. The plastron in males is noticably concave (some females may have a mild concavity), the hing legs longer, the face longer and more pointed, the shell less rounded, the inner toe on the hind foot will be turned inward more than on a female, and the eyes are typically red. Some females may show red eyes, but when you compare all of these traits, especially with a male and female in hand, you can usually tell the difference. Three-toed box turtles are the one exception to this rule - they are natoriously difficult for identifying gender.
As for snapping turtles, I can't help you there. I haven't seen enough individuals in one sitting to be able to judge.
Katrina
