Here's a good article on spaghetti worms, pics and diagrams near the bottom:
http://reefkeeping.com/issues/2002-06/rs/index.htm
If you do have flatworms, I had some for a while when I first set up my reef. Tried a couple of things, none of them worked. Then, when I just said "screw it," they slowly started dwindling. I haven't seen any in about 10 months. No idea why.