Many years ago, when I was just learning about wild edibles, I was so intrigued with the notion of eating fiddleheads that I carelessly misidentified the following fern for the edible Fiddlehead Fern (Matteuccia struthiopteris, often called Ostrich Fern).
Luckily I realized my error before sitting down to eat the bowlful I'd gathered and boiled. That experience forced me to stop guessing and take plant identification more seriously. Eventually, I correctly identified this fern as Cinnamon Fern (Osmundastrum cinnamomea). While both Cinnamon Fern (CF) and Fiddlehead Fern (FF) are large ferns that grow in circular clumps, there are ways to tell the two species apart, even when they are just unfurling. For one thing, the fiddleheads of CF are covered with wooly coats (see above), while FF features papery scales. Secondly, CF stalks are mostly round, with one flat side, whereas FF stalks are grooved like celery.
Once fully unfurled, wooly remnants on the lower parts of CF and the grooved stalks of FF remain helpful keys to identification -- though, with any plant identification, it's best not to rely on a single characteristic. For proper ID, I recommend consulting a fern aficionado or using a reliable field guide key, like the one in Peterson Field Guide to Ferns (2nd edition).
I now know that Fiddlehead Fern is absent from my immediate surroundings (FF commonly inhabits the floodplains of major rivers, which Plainville happens to lack), and that the inedible, though no-less-important, Cinnamon Fern grows in abundance in moist woods and along streams throughout town.
Note: If you are interested in seeing this fern first-hand, join me on Sunday 6/23 for a free ferns and feathered friends walk in Wrentham, MA.