This brings up a good question -- which trails are commonly skied? Most of the hiking trails I've been on don't seem condusive to skiing. But I'm not a skier, so I don't have any real sense of what is considered ski-able.
Well there isn't a formal list, of course. Obviously, seeing tracks, skiers, or consulting one of the best-places-to-backcountry-ski books are ways to make a list.