Turtle Gender

In the case of most aquatic turtle breeds, it can be extremely difficult to determine if they are male or female, until their shell is about 4” long.   At this point, you should start to notice a few differences, and these differences will become more obvious, the older/bigger your turtle gets:

 

The Claws—With many aquatic turtle breeds, the males will have much longer front claws than the females.   

The Cloaca-- This is the hole from which turtles eliminate (go to the bathroom), mate / have sex, and lay eggs from (for females).    In a female, the cloaca is closer to her shell.   In the male, it is further away from the shell.  here are photo examples of RES's (Red Eared Slifer turtles), but they are the same in most aquatic breeds:

The Bottom Shell (Plastron)--  Male and female turtles are usually slightly different in their bottom shell / plastron.  The females plastron will be more flat, or even bulge out slightly.  The male’s plastron will dent inward a bit—this is meant to help him “fit” over the females rounded back shell during mating.   

The Top Shell (Carapace)—The female’s top shell / carapace is often slightly higher and more “dome-like” than the males.   This is to give her more room to hold her eggs.   Unfortunately, this will be almost unnoticeable, if they are young enough to be the same size...

Head--  The male's head (especially the nose is more long, skinny and pointed.  The females is more blunt.   

The Tail—Males usually have a thicker, longer tail.  While females have a skinnier, shorter tail. 

These are the visual differences for many smaller, more common aquatic breeds.  

Age / Size— As your turtles age, you will notice that the female is larger than the male.  Some can even be almost twice the size. 

But if you have a larger or less common breed (such as a soft shell, a box turtle, or a tortoise), then there may be some other differences that are specific to that breed.  You can look them up on Google.