Note the time and location in which you found the caterpillars in a notebook. Include the kind of plant on which you located them, the geographic location and the month. If the caterpillars are eating anything, note what it is.
Describe the caterpillar in your notebook, including the size, any colors or patterns on its black surface, its fur or lack of fur, spines or any other distinguishing marks. Sketch a picture using a pencil if you can. If you have a camera, take a photo.
Contact the department of biology at a local college or university to find out if there are any entomologists working at the school who could help you identify the caterpillars. If no one is available with the necessary expertise, contact a local entomological society or a registered extermination company. If the species is a pest, exterminators will likely recognize it.
Check the pictures and descriptions of different kinds of caterpillars listed on the "Butterfly and Moth Caterpillars Identification Guide," the "Moth Photographers: Larvae of North American Lepidoptera" listing, or in the listings of different caterpillars found in the "Caterpillars of Eastern Forests" website. Check the descriptions and photos to find a caterpillar that matches the habitat, diet and description of the one you are analyzing.
Contact the administrators of those websites. Email them a description of your caterpillar, including all the information you noted, along with any photos you may have.