Oooh, congrats. Exciting eh? Well, when the spid is off his old skin, take it out and open up the abdomen, carefully!! Between the 2 white bits by where the carapace meets the abdomen there will be spermathecae (spelling?). If you can't see anything there, the chances are it'll be a male, as it's what used to store sperm.
I'm sure someone else will explain better than me! :blush: