If it smells worse than rotten eggs, and falls out of the shell easily it is most likely dead. If you touch it and it sucks itself back into it's shell, it is still alive!
TOUCH IT IF IT MOVES OR BUBBLES UP ITS ALIVE
Not necessarily as lots of snails float from time to time. Pick it up and touch the foot with your finger and see if it reacts.
The only way to tell if a snail is dead is to sniff the snail, if it gives off a strong, odorous scent, it is most likely dead.
It will stink like dead fish.
if it hasn't moved in a week it is probably dead
the amount of salt dissolved in the water
its shell drops off its body, or it lays upside-down on the bottom of the tank
Snails are hermaphrodites, so they are both male and female.
The density of a salt water solution compared against the densities of "pure" water and salt provides an approximate value of the ratio of water to salt in the mixture.
Salt is the solute. Water is the solvent. Salt water is the solution. This solution is sometimes called a saline solution.
taste
Snails that do not move for a day are definitely not well. An easy way to tell is to pick it up and smell it, and if it smells rotten, then its dead or dieing. Don't inhale too deeply, as rotting snail smell is one of the most disgusting smells you can smell.