Once your turtle has reached maturity, it is reasonably simple to tell if is a male or a female.
Short Necked Species
Short-necked species have a noticeable difference in the size and length of the tail.
Males (above) have a large, thick tail. When the tail is relaxed, the cloaca is situated behind the rim of the shell.
Females (above) have a smaller, thinner tail. The cloaca is close to the base of the tail.
Long Necked Species
Male Eastern Snake-Necked turtles may have a long tail, when compared to a female. But some females have a large tail when compared to a male. In reality, you need to actually observe what comes out of the tail to be sure if the turtle is male of female. If you observe the turtle nesting, such as the one below, then obviously it is a female.
Or, sometimes a male turtle will leave you in no doubt as to his gender, such as this one.
One popular method of distinguishing males from females relies on the shape of the notch around the tail. Some males have a 'V' shaped notch and some females have a 'U' shaped notch. For Eastern Snake-Necked turtles, this is not always a reliable method.
There are males with a 'U' shaped notch and females with a 'V' shaped notch. In the following photos, the left hand and centre turtles have a 'V' shaped notch. Yet all five turtles are female.
An alternative (but not 100% reliable) method of distinguishing males from females is to place a straight edge across the plastron at the point where the Anal Shields and Femoral Shields meet, as shown by the blue line in the diagram below.
© 1978, John Cann. Reproduced with permission.
If the straight edge across the plastron rocks from side to side (convex plastron) the turtle is usually a female (see below).
If the straight edge shows daylight along the centre of the plastron (concave plastron) the turtle is usually a male (see below).