There's a distinct way of knowing if you've got the right one. They have a U-shaped stem. As far as I know, there's only one edible fiddlehead. It's an ostrich fern. I've seen another fern that reddish and has a twig-like round stem. Those aren't edible. Yes, they have to be cooked about 15 minutes. Blanch for 2 minutes before freezing them.
Read this Health Canada link:
http://www.hc-sc.gc.ca/fn-an/securit/kitchen-cuisine/fiddlehead-fougere-eng.php