Mystery Snails are not hermaphrodites like some other snail species. There is a male and a female. They can not change sexes.
You should wait until the snail is quarter size or larger to sex. Sexing smaller sizes will likely lead to sexing incorrectly.
Males will have a visible penis sheath on the inside of their right shoulder. Do not mistake their poop chute for their penis sheath. Their air siphon on their left shoulder is often also mistaken for their penis sheath.
To sex them yourself simply hold the Mystery Snail upside down. They hate being upside down and will open up to try and flip themselves over. Be patient some snails are more shy then others.