They don't have genders.
You simply do not. There are no such thing as male and female snails.
All snails are hermaphrodites.
That means all snails have both male and female reproductive organs. They still need to mate with another snail in order to produce and lay eggs.
Some amateur gardener groups will try to say that snails with a brown-striped shell are girls whereas white stripes are boys. This is false - they are actually two completely different species. Therefore "snail breeders" breeding these two are creating a hybrid or mixed-species.
So the answer to your question is that you can not tell if a snail is male or female, because snails do not have male/female genders. They are all hermaphrodites.
Most snails are hermaphrodites (both male and female). Usually during mating the larger snail will be the female and the smaller one will be the male. If both snails are the same size it is quite likely that they will both carry eggs. But a common snail in the aquarium trade is the apple snail. It is either male or female. Refer to the picture in the link for a diagram of a male and female specimen of this species.