I have noticed they do have some odd behaviors that other snails so not have.
1) They sometimes spend ALOT Of time inside the shell. When this happens, try a water change. usually after a WC, they come out of their shell. I had to add some coral to my tank - to harden the water a bit. Otherwise, a day or two after a water change - they were back in ther shells again
2) They spend a good deal of time laying on their back with their meaty parts hanging out. Alot of people would assume its dead, but they are not. Not sure what they are doing, maybe just chillin?
3) When smaller, they float on the surface. If you watch them, you can see they are eating whatever is on the water surface.
I have also heard that alot of people have issues keeping them alive - I beleive it is because they are not feeding the snails. They will eat algae but not all tanks have enough algae in them. They grow fast in my sisters tank becasue she has goldfish and feeds them too many flakes and they eat the flakes the goldfish miss. I should also mention that she has 2 goldfish in a 20G. God only knows how high the nitrates are in there since SHE never changes the water. I do it but about 30% a week which is not enough.
you will now when its dead becasue the trapdoor will no longer be attached or it will easily come off it you try and remove it.
Apple snails/mystery snails are also easy to care for - but they like warmer waters. They can get pretty big too. applesnail.net will tell you all you ever wanted to know about them
That one is about 4 months old and is about as big as an adult trapdoor snail