Tomato plants have distinctive stems, vines, leaves, flowers and fruit. The most distinctive part of a tomato plant, however, may well be its scent. Michigan State University Biochemistry and Molecular Biology Professor Robert Last identified two genes and enzymes present in tomato plants that may actually repel insects that might otherwise seek to prey on them. It is not an easy scent to describe, but all tomato plants have it, regardless of variety. Once you smell it, you will not forget it.
Note the stems of the plant and whether they have short, fine, white hairs on them. Tomato plant stems are slightly fuzzy. Observe the plant's growth structure as well; tomato plants, even determinate ones, have a vining habit.
Note whether the leaves are green, have visible veins, are fuzzy and are compound. These traits are all present on tomato plants.
Note whether there is a strong, distinctive smell emanating from the plants if they are disturbed. Note also that flowers on the plant, if there are any, will not contribute significantly to the scent.
Observe any flowers on the plant. Tomato flowers are bright yellow, with pointed petals.
Take note of any fruit growing on the plant. All tomatoes start out as a single small, round, green fruit that begins growing when a flower is pollinated. Most tomatoes change color only as they ripen.