Pdq-- Ostrich ferns are distinguished by the deep groove in their stems (i.e. if you break one off, the cross-section will look like a "u", not a "D" or an "O") a covering of dry brown papery scales, and an absence of any hairy or woolly covering.

While bracken fern and most other species are slightly toxic to humans, the risk they pose is nowhere near the risk posed by a mis-identified mushroom.

Sorcha-- I've been enjoying fiddleheads here in Maine for years, but I always end up getting them at the market unless someone gives us some as a gift. They have been variously described as tasting like asparagus, broccoli, okra, and/or mushrooms. I think they're mostly like asparagus, myself, and I prepare them the same way: wash well, steam until slightly past the "al dente" stage, then serve with butter and a little bit of cider vinegar or balsamic vinegar.