Have you seen this page? http://www.tronchaser.com/~cindi/ss.html
It may help you when trying to sex them. I had the same problem you're having. I wanted to sex mine because I'm giving a couple of them away and want to make sure a keep a pair that can mate later on.
Here's a picture of a mystery snail (p. bridgesi) with it's siphon extended on the left side of it's head. This isn't my snail. I googled "mystery snail siphon." The siphons on mine look just like this one's.
mine has the same siphon. Sort of cone-shaped, with yellow speckles on it, on the snail's left. So the thing over on the right (in my pics) must be the end of the digestive tract. I think I know what to look for now.
Thanks all, for your help! Off to look for a boy snail!
The second photo is also good. The white thing more on the right is the penis and the left thing is his siphon. The penis is next to the poop hole, if I can recall, just more interior to the midline of the snail.