Identify a caterpillar by its main body color and any patterns it has, whether it has hair and how dense the hair is. It can also be identified by features such as lashes, head horns, tails, split tails or knobs. Other caterpillars can be identified by their spines or tentacles.
Knowing the caterpillar's host plant also helps to identify it. In some cases, the appearance of the caterpillar changes as it ages, as in the monarch butterfly.
Use a nature guide or other online resource to identify the caterpillar. For example, one type of caterpillar becomes the banded sphinx moth. The caterpillar of this moth is green, banded and hairless. It has no other distinctive features.
The cecropia moth caterpillar is also green, but it is robust and hairless, with tell-tale knobs over its body. If the person knows the name of the butterfly but does not know what the caterpillar looks like, he can look up the butterfly in a nature book or online.
Often, there is a picture or description of both the butterfly and the caterpillar on nature or specialized caterpillar websites. The sites may also point out valuable information about color and host plant descriptions; for example, the monarch butterfly begins as a caterpillar with black, yellow and white stripes and two pairs of tentacles, and the caterpillar's host plant is the milkweed.