if he is dying, he will be expelling his insides.... and seem to be shrivled up. also if you bring the anemone to the top of the tank and give it a smell test (horrible smell being a sign of death) that will help you too.
as far as giving off pollutants...i've heard mixed things. but IMO they would give off nothing good. considering dying organisms decompose which may restart your cycle... don't quote, just my opinion.