Apple trees are easily identified by two distinguishing features. The first is that they have characteristic leaves: the face of the leaf almost always upward, and the edges of the leaf also curl upwards, revealing the lighter shade of green on the backside of the leaf.
The second feature is that apple trees are just kind of scraggly. They tend to look like large bushes instead of trees, and they require a lot of pruning. Unpruned trees (as we find 99% of the time) will have a large amount of branches and twigs growing on the inside of the tree. They will also often have many small patches of brown, dead leaves which are easily visible in contrast to the rest of the tree.