The easiest way to identify a fruit tree is to wait for the fruit to grow and see what you get. Use a tree identification guide to identify it based on its leaf shape or type of blossoms.
Leaf shape, leaf arrangement, number of petals and type of bark can help narrow down the identification. Height can also play a role, since some varieties of fruit trees look very similar but grow to different heights.
Your location can also help narrow down the type of fruit tree. The U.S. Department of Agriculture divides the country into various zones based on climate, and not all trees can grow in all zones. Citrus trees and olives are notoriously limited and can be distinguished by their evergreen leaves.
If you are still struggling, try asking your local nursery for help. Many keep detailed guides on hand, and the experienced employees may notice details you do not. Online tree identification guides with detailed pictures or the smartphone app Leafsnap may also help. Leafsnap uses pictures from a phone's camera to identify trees by the shape of the leaf. However, it is limited to trees in the Northeastern United States and Canada as of 2015.