Shortnecks are relatively easy, males tail is longer and thicker than females.
Longnecks, similar, males tail is slightly longer and thinner at the tip, females cloaca is closer to the end of the tail. The "v" shaped notch in the plastron (lower shell) is more triangular in females and slightly more "u" shaped in males. It helps to have a few for comparison.
heres a great caresheet http://www.vhs.com.au/pages/CareSheet_FreshWaterTurtles.html