How do I know if my snail is dead?
 

It just doesn't look right.

If you take it out of the water, it will SMELL. Also, a dead snail will not be able to keep the door shut.

Well, you could just wait a while. If it doesn't move at all in several hours, or in 12-24 hours I'd strongly suspect it was dead.
Down side is, if it's dead, your water would be getting fouled...

other fish / inverts will be eating it? ;)

When a snail dies the muscles relax, and the big muscle that contracts to close the "door" on the shell wont be able to stay shut, and it will be slightly open.

Also, you can gently touch the snail and see if it moves or retracts itself.

Snails sometimes don't move for VERY long periods of time. Not moving is not an indication of death. Like lascolinas mentioned, it will react and close it's door if it's alive. If the door doesn;t close despite handling it, it's probably dead.

Also, a dead snail smells awful. Pull it from the water and give it a sniff. If it stinks, it's dead. :-(

Exactly I had a preety snail and I pulled it out for a cheackover (I cheack my big ones for things on the shell) And it stunk and I knew that it was dead:-(

The shell and foot were lighter colored,it stayed open slightly and on it's side overnight in a Betta cup, didn't retract, and smelled bad. My mom picked it up to give a second opinion and yep it was a goner. At least this can be a reference for anyone else. This snail however seemed to do nothing but hang out at the water line in the four weeks I had it. It never seemed to do any more than move from one side of the tank to the other. My golden constantly cruises though. Did I pick a snail with a problem from the start?
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Quite possibly.