Mystery snails are more commonly known as apple snails and are aquatic pets found in freshwater aquariums. They are generally hardy snails that can often survive in poor quality environments, but are prone to sudden death from disease or old age like any other pet. Identifying a dead mystery snail can be difficult because the animals will sometimes float on the surface or be inactive for days on end. You will need a little patience and a keen sense of smell to tell if your snail is dead after a week or so of inactivity.
Remove the snail and smell the foot. The foot is the small, flat portion right below the snail's body that acts as a "door" to the shell. The body will have a pungent, rotting odor if the snail is dead. They are normally odorless, although poor water quality could lead to a bad smell.
Pull the foot back gently. The snail is still alive if it retracts its foot back into its shell.
Set the snail on a plastic sheet outside the tank and watch it diligently. It will open its shell and begin to move around in search of water after a few minutes if alive. They have lungs that allow them to breathe outside of water, so the snail will not be harmed.