I can only speak from my experience of losing a bubbletip rose anemone, due to my own ignorance when I first started keeping saltwater tank. An anemone that has expelled and deflated will still be attached to rock or substrate with it's base. It will still respond to a light touch, although not near as much as when it is fully inflated. As mentioned, it will re-inflate with water and appear normal within a few hours.
A dead anemone will normally release it's "grip" on whatever it was attached to, when you touch it you will see no response, and it will appear to be falling apart. By this time it will also begin to smell, and should be removed pronto.