Take your chosen mystery snail out of the water (don’t worry, they have a lung and can breathe air — this does not harm them at all as long as they aren’t kept out of the water for a long period of time).
Sit down and hold the mystery snail on its back between your thumb and forefinger in a slightly vertical position.
Wait… wait some more… wait some more. In all sincerity this takes an incredible amount of patience because the snail could come out in 5 minutes, 30 minutes or not at all.
Gender can be identified when the snail comes far out of its shell (see photos below for how far).
Look at the snail’s RIGHT shoulder. (The left side of each image as you look at the photos on your screen)
On a female you will see what looks like two holes on either shoulder just under the shell.
On a male, you will see the sheath mostly blocking the hole on the snail’s right shoulder side (left as you look at the photo on your screen).