There are about 100 trees and shrubs in the birch family (Betulaceae) and about 40 known species of birch (Betula), according to C. Frank Brockman in his book "Trees of North America." To identify a particular species of birch, you will need to use a tree guidebook for the region in which you are located. However, there are some characteristics shared by the entire birch species that will enable you to at least distinguish birches from other trees.
Look at the leaves. All members of the birch family have leaves that are deciduous (regrow each year) and alternate (grow staggered on the branch and do not grow in symmetrical pairs along the stems as some other species).
Observe the leaves up close. Though different species of birch have different shaped leaves, they all have prominent veins, noticeably toothed margins and short stems. In the fall they are likely to turn yellow as opposed to other deciduous trees, which may turn a variety of red or orange hues.
Examine the bark. In some species, such as the paper birch, the bark peels in a papery horizontal fashion that may curl into long strips. Many species of birch do not have peeling bark, but they all have horizontal lenticels (spongy, dark lines in the bark). To the touch, birch bark feels thinner compared to many other common deciduous trees such as oak, maple and ash, which have rougher, thicker bark.
Look for the brittle, cone-like catkins (or fruit) of the birch in spring. The staminate and pistillate flowers (male and female flowers) are in different stages of bloom on the same tree. Staminate (male) catkins are developed the previous season.
Look for the tassel-like staminate catkins in early spring. These are anywhere from 1 to 4 inches long and, when ripe, scatter many scales and seeds that make up the catkin. Whole, they look like little, brown, elongated cones that are more cylindrical than round.