Most land snails, or pulmonates, are hermaphrodites, meaning they possess both male and female sexual organs. However, land-oriented prosobranch snails, as well as many water snails, are dioecious, meaning they can be either male or female. The apple snail and the tiny New Zealand mud snail, for example, are dioecious. If your snail is of the dioecious variety, determining its sex will take some patience and careful observation.
Remove the snail from the aquarium or other habitat, gently holding it between your thumb and forefinger.
Keep the snail on its back, belly facing the sky, for one to 10 minutes.
Watch for the snail's foot, a wide, flat membrane, which it will extend in order to try and right itself.
Look through the magnifying glass as the foot extends, training your vision on the right mantle cavity, attached to the inside of the shell over the head and neck.
Keep your eyes peeled for the snail's relatively large penis sheath. It will resemble the front of a pouch. If you don't see one, the snail is female.
Practice observing different snails to improve your gender-recognition skills. It may take a few tries before you can tell the difference.