Classify the plant. Is it a tree, a bush or a vine? Do the leaves grow on a vertical trunk, a creeping vine or in clumps on a shrub? Check to see if there are lots of other similar plants close by, or if it is the only one in the area. Especially with young plants, it may be difficult to distinguish what category of plant it is, but having an idea of how the plant grows will help you identify the leaf.
Take a sample leaf from the plant. You'll want to be able to look at it and study it while you identify it.
Look at the shape of the leaf. Are the leaves needles like an evergreen, or are they flat and hard like scales? Does it have broad, flat leaves or long, narrow leaves?
Study the growth of the leaves. If it's a broad leaf, check to see if it is a compound leaf, with many different leaves on one stem. Or is it a simple leaf, with just one leaf on a single stem? If the leaves are needles, do they grow in clusters or singly?
Check the shape of the leaf. If it is a compound leaf, check to see if the leaves grow from one spot or from different places along the stem. If it is a single leaf, does it have lobes on both sides, and if so, are they balanced on both sides or irregular? If needles in clusters, how many needles are in each cluster? If flat, does the leaf have a smooth edge or a serrated edge?
Use a leaf identification website or book for your region to identify the leaf. The leaf identification tool should be organized into different types of leaves. Now that you have identified many of the characteristics of your leaf, you can quickly find its match in a classification tool.
Compare your leaf to pictures of the plant you think it is to confirm identification. Once you think you know what plant it is, look at several pictures to make sure that the plant you are identifying matches in shape, color and growth.