I believe I have a lemon tree, that gives very small sized fruits. They are 1 and a half inches to 2 inches at most. I think they are lemons. The fruits drop when they are mostly green and yellow at best one third of the way. They will turn yellow when left on the ground.

Today, a fellow worker of mine brought some large sized limes, but they look like lemons. They are about 3 inches long and look lot like lemons. I open both my citrus fruit and his fruits and smelled them, but I cannot tell the difference. I guess I'm not good at distinguishing them.

Is there a some method in distinguishing the lemons from limes?

There are two fruit commonly called "lime" in the English-speaking world. 'Key' lime, a.k.a. Mexican or Indian lime, is the smallest of the bunch -- fruit around the size of a pingpong ball or even smaller, turning yellow upon ripeness. Almost always with a few seeds, which are small (1/2 the size of orange seeds). Quite a thin rind. Trees are small, with tiny leaves compared to other citrus. New growth is green. Flowers are small (2/3 the size of an orange flower) and white.

'Tahiti' lime (a.k.a. Bearss, Persian lime) is closer to the size of a lemon, although usually not quite as large. Fruits may have a bit of a nipple at the blossom end, but usually not a neck at the stem end. Marketed when dark green, but they will turn yellow if left on the tree until mature. Virtually always seedless (averaging less than 1 seed per 1000 fruit). Leaves are average size (like those of an orange), with a short but rather broad, highly variable petiole wing. Average size tree. Fruit pulp tends to be pale green to almost clear. New growth starts out pale green then turns darker green. Flowers are pure white with greenish (empty) stamens.

True lemons make larger fruits, usually with a "neck" and a "nipple" at the ends to give them the classic lemon shape. Most will have at least one seed (Eureka) or many (Lisbon). Seeds are about the same size as an orange seed. Leaves are among the largest of all citrus (twice the size of an orange leaf) with no petiole wing at all. Trees grow to be among the largest of all citrus. Fruit pulp tends to be pale yellow to almost clear. New growth on the tree is noticeably coppery or purple until the leaves expand. Flowers have purple on the backs of the petals, white on the front side, with yellow stamens full of pollen.

Lemon peel oil, as well as lemon leaf oil, will smell strongly of lemon. The lime oils, while fragrant, definitely smell different from the lemons.

'Meyer' lemon (which is a lemon x orange hybrid) makes the largest fruit of the bunch, often more rounded and lacking the nipple (and perhaps no neck). medium size leaves without a petiole wing on a small- to medium- size tree. peel and leaf oils have a unique fragrance, close to but not the same as that of a true lemon. Fruit have a thinner peel than a true lemon and tend to be more juicy and less acidic. New growth may show some color, but usually not the bright, deep copper or purple of a true lemon. Meyer flowers may have some purple on the back sides of the petals.