When some limes have not yet ripened, their color may resemble the yellow color of a mature lemon. Also, some citrus trees produce yellow limes, called "key limes." Some limes can be shaped like lemons and turn yellow when they fully mature. Although it can be difficult to determine by color whether or not a citrus tree is bearing lemons or limes, there are other characteristics of the tree that will help you determine its type.
Examine the size of the tree. Generally, lime trees are smaller than lemon trees. Lime trees grow to be 6 to 13 feet tall. Lemon trees, on the other hand, will grow to be 10 to 20 feet tall.
Look at the leaves. When the flowers are in bloom, the flowers of a lemon tree will have purple blush, while the flowers of a lime tree have white blush. Also, most lemon tree leaves will be larger in size than lime trees, ranging from 4 to 5 inches in length. Most lime tree leaves are no more than 3 inches long.
Smell a leaf. When torn, lemon leafs have a mild scent. Lime tree leaves, however, have a strong lime scent that immediately identifies the tree.
Smell a sample of the tree bark. Carefully break off a leaf and smell the exposed bark underneath. The bark of a lemon tree has no perceptible scent, while the bark of a lime tree has a very strong lime scent.