It's very hard to sex them because they all look the same (size, color). I sex my snails by who's on who... but if you really want to know, you can take the apple snail out and wait for it open up. On the left, you should either see a penis sheath. Otherwise you should see an opening, which this would be a female. erm... perhaps it's easier if you google images. Here:
In addition, if you want to breed these creatures, make sure you have lots of calcium dissolved in your water. Eggshells, cuttlebones, and old snail shells works great.