Find a good tree identification field guide, such as "What Tree is That?" published by the Arbor Day Foundation. Or get a good start with its online leaf identification tool, which you can use for free at Arborday.org.
Pluck several leaves from each fruit tree. Find leaves that are large and dark green rather; avoid unformed new growth. Make sure the leaves are free of tears, brown edges or holes that obscure their features.
Lay the leaves out on a table. Note the features of each one. Is the leaf a single, large, flat leaf or is it comprised of several smaller leaves? Are the edges rough or smooth? Are they tapered or fan-shaped with several spiky lobes (projections on a single leaf)?
Consult your leaf identification book or online identification guide. Any good tree identification resource should tell you what to look for and in what order. You'll begin narrowing down the type of leaves by looking at general outline, then edge ridges, then the shape of the tips of the leaves, and so on.
Narrow your options until you find a picture that looks exactly like the leaf on the table. All leaves vary, but the leaves from one tree will have specific visible characteristics. Match little details like leaf-vein patterns and the texture on the leaves' undersides.
Identify peach leaves by looking for trees with long, narrow, sharply-pointed leaves. The leaves will have very fine teeth all around the edges and be from three to 5 inches long. Compare leaves like this with photos and descriptions in your tree guide to see which type of peach tree you have.