Some trees grow in many different regions, but others grow in specific habitats. Field guides and online identification sites often ask for the location of the tree. At the Arbor Day Foundation website, you must choose between "Eastern and Central United States" and "Western United States" to start the identification process. If you are not sure where the tree grows, refer to a general field guide rather than a regional one.
The distinction between evergreen and deciduous is a major factor in identifying trees. Trees that keep their leaves all year are called evergreen. Some evergreen trees have slender leaves like needles, while others have leaves that resemble fish scales and are often on a thick stem. Trees that shed their leaves each year are called deciduous. Their leaves are thin, flat and attach to the stems individually.
Bark is the easiest part of a tree to examine since it is close to the ground and accessible all year, unlike leaves which may be missing during the winter. Compare the color and texture of the bark in the photograph to what you see in the field guide or online. According to the University of Florida Extension, bark is found "in many shades of white, gray, brown, red, purple, green, and yellow." Bark texture ranges from smooth to deeply furrowed and can be flaky or warty.