These days, you are more likely to run across Slippery Elm (Ulmus rubra) in the woods than American Elm (Ulmus americana), but it's pretty difficult to tell them apart in the winter. The easiest way to separate them is by looking a the samaras, which should be developing in a few weeks.
The samaras of American Elm have fuzzy hairs along the edges of the wings, while the samaras of Slippery Elm are relatively smooth.
http://plants.usda.gov/java/la...
http://plants.usda.gov/java/la...
