I've only seen one species in person. Some starfish (like the crown of thorns) are extremely large...and eat coral. There are plenty of starfish that eat coral...including the chocolate chip starfish, which is medium sized.
If a starfish is missing a leg...or has more than "normal"...it's normal. There doesn't seem to be a good indicator at all as to which of the small "asterina" starfish will eat coral and which won't. If you see a starfish on the healthy flesh of your coral...just pick it off. Otherwise, you're probably fine.