Mystery snails are all the same species, so they can and will breed. You can find out about sexing snails on applesnail.net, but the only way I have experience with is waiting til they mate (assuming there's a male and female present)
http://www.applesnail.net/content/anatomy/reproduction.php shows the difference between males and females, the penissheath on the male. It's not the easiest thing to check though. I never could tell with mine and I never caught them laying eggs in the act so I never could tell which of my three was a female. If you see them in the mating process, the male is on the females shell.