Lemons and limes may both be sour--though sweet variations do exist, they're relatively uncommon in North American gardens--but they are completely different fruit. If you are growing both types of citrus in your yard, you may forget which tree is which, especially when the trees are not bearing fruit. A few investigative strategies can help you compare the two to find the difference between a lemon tree and a lime tree.
Break off a leaf from the tree and rip it in half. Sniff the torn pieces. You will notice a lime-like smell on leaves from a lime tree, while lemon leaves have a mild or indistinguishable citrus smell.
Look at the tree's fruit. Even before they turn their trademark yellow, unripe lemon fruit are often more pointed in shape than lime fruit.
Examine the tree's flowers. If you see flowers with a purplish tint in their centers or edges, it's a lemon tree. Lime flowers are mostly white.
Compare tree and leaf sizes, though this can be an unreliable indicator because mutations or hybrid species can break from tradition. Typically, lemon trees grow taller--15 to 25 feet in height--than lime trees, which often max out at 12 to 15 feet. In addition, the leaves of a lemon often grow bigger, at four to five inches compared to a lime tree's two to three inch leaves.