you'll want to check her shell for any signs of irregular growth, holes, pits, cracks and other deformities. apple snails do enjoy a good rest once in a while which is normal. you might even see them floating at the surface, sometimes for days. then before you even notice, they are back on the glass and eating away. if your really paranoid, just take the snail out of the water. if it smells really bad, its probably or more likely dead. if the operculum (shell door) is completely shut, then its resting because a dead snail would have relaxed muscles and therefor the trap door would not completely close. poke at it gently and see if they squirm or move around.