Every tree can be identified by its leaves. There are defining characteristics that are unique to each tree, and while it may be hard to pinpoint an exact cultivar of a specific type of tree, you can at least get a good idea what family the tree belongs in without taking the leaf to a professional. Careful observations, though, are necessary to correctly identify the leaf. Fortunately, with the Internet, identifying what kind a tree a leaf belongs is now easy.
Observe characteristics of needle-like leaves. Some trees grow single needles for the stems but others have bunches. Count how many needles grow in a bunch, if applicable. Some needles are flat and some are square, round or scaly. Measure how long the needles are and whether they are thick or thin. Take note of any other defining characteristics that may help with identification.
Observe how broadleaf or flat leaves grow from the branches. Do two leaves grow opposite each other from the branches or do they alternate along a branch-- one growing from one side, and the next leaf from from the other side, and so forth?
Look at the broadleaf to see whether it is a compound leaf -- several leaflets that grow from the same leaf stem. On the other hand, it may be a simple leaf -- only one leaf per leaf stem.
Look at other characteristics of the broadleaf. Is it lobed, smooth, shiny, dull, toothed, round, oval, shaped like a heart or hairy? Are the lobes pointy or rounded? Are the teeth pointy or double? Is the leaf an evergreen or deciduous? Is it symmetrical?
Compare the characteristics of the leaf you examined to the information found in a garden book, tree field guide or online on a nursery or county extension website. Several leaf identification guides or dichotomous keys are also online. There, a series of questions narrows the search until it identifies the leaf.