Note whether the fern remains green during the winter or dies back. The maidenhair (Dryopteris marginalis) and interrupted ferns (Osmunda claytoniana) are deciduous, meaning that they die back in the winter. The evergreen shield or marginal wood fern (Dryopteris marginalis) and the Christmas fern (Polystichum acrostichoides) stay green throughout the year.
Feel the leaves. Even if you don't know if the fern survives the winter, the evergreen ferns tend to have leathery foliage that prevents them from losing water during the winter months. If the foliage feels leathery, you likely have a marginal wood or Christmas fern.
Observe the shape of the plant. According to the Native Plant Information Network, the maidenhair fern has the unique characteristic, among native ferns, of its foliage spreading in a circular pattern around the center of the fern, giving it a fan-shaped appearance. The marginal wood and interrupted ferns' fronds grow upward before bending outward, giving them a vase shape. The fronds of the Christmas fern grow in clumps.
Examine the foliage. While ferns all have distinctive feathery leaflets, the leaflets of some species further subdivide, often multiple times, into additional leaflets. The marginal wood fern is twice-pinnate, meaning that the leaflets are subdivided once into additional leaflets. The maidenhair and Christmas ferns have only single leaflets, while the interrupted fern has single leaflets except for fertile fronds arising from the center of the plant, which are twice-pinnate.
Estimate the size of the fern. While most Pennsylvania native ferns remain relatively small, reaching no more than 3 feet in height, the interrupted fern can grow up to 6 feet tall.
Find a fiddlehead, if you're observing the fern in the spring. Christmas ferns have silvery fiddleheads, while the fiddleheads of the maidenhair and marginal wood ferns are dark red and golden brown, respectively.