actually mid 70's would be a better temperature for her. Apples/mysteries can "Age out" when kept at too high a temperature.
There is a way to sex them. But I've never been able to.
the most reliable (though still not totally accurate) way to sex a mystery is to look at the front lip of their shell. if it appears to be an even curve (like yours) they're most likely female, whereas a male will have a bit of a peak in it, kinda like the tail on a water droplet. other than that, watch their behavior. males have a habit of, erm, "enjoying themselves" over any surface they can find, particularly if you've got a rock or decoration that has a round knobby end on it that sorta approximates the shape of another snail's shell. if you have multiple mysteries, this will become more apparent, since they tend to mate with each other ferociously, whether the other participant is male or female. (take THAT, fundies who say that same-sex relations are "unnatural!")
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)