The most efficient and accurate way to identify tree species is usually by studying their leaves, flowers and any fruits. In winter, you don't have this option for most trees. The first and easiest step in identifying trees in the wintertime is noticing whether the tree has lost its leaves, meaning it's deciduous, or if it still has its leaves, meaning that it's evergreen. If the tree is evergreen and a conifer (needle-leaved), you can determine the tree's exact species by studying the size and arrangement of the needles, as well as the tree's overall size. For deciduous trees, you'll need to study other characteristics in order to identify it, such as the bark, twigs, buds and branching formations.
Study the tree's branching formation to determine whether it's alternate or opposite. Dogwoods, ashes, maples, Viburnums and horse chestnuts have opposite branching, where the limbs and twigs grow off the main branch in pairs. Alternate-branching trees, such as sycamores, birches and tulip trees, have twigs and limbs that grow off the main branches singularly and not opposite each other.
Look at the twigs to study where the leaf scars are located. The leaf scars can tell you where the leaves grow and how they're arranged on the twigs. Also notice the thickness of the twigs. Walnut trees have fewer but thicker twigs, while elms and maples have numerous thin twigs.
Identify the tree by looking at its buds, which are very distinct in each tree species. For example, linden (basswood) trees have double red buds, silver maples have clustered red buds, bitternut hickories have yellow buds, horse chestnuts have sticky buds and tulip trees have duckbill-shaped buds.
Study the bark to identify trees in winter, especially sycamores, white birches and chestnuts and certain species of hickory trees. White ash trees have ridged bark in a diamond-like pattern, most young maples have noticeably smooth bark, black cherry trees have dark small-plated bark that looks like chips, red oaks have rough-textured vertical ridges on the bark, and shagbark hickories have large, vertically-peeling thick strips of bark.
Identify trees in the wintertime by smell. Snap a twig to smell the inner pith or peel back a piece of the outer bark. Sassafras trees have a strong spicy but pleasant smell, wild cherry trees have a bitter almond-like odor and sugar maples have a slightly unpleasant odor.