How do I identify a caterpillar?
Some readily available books are the Peterson First Guide to Caterpillars (Amy Wright), the Golden guide to Butterflies and Moths (Robert T. Mitchell), and Caterpillars (Chris Earley).
There are a few good resources available on the Internet, but there are thousands of kinds of moths and over 700 kinds of butterflies in North America and only a small portion of the caterpillars are illustrated in these sources. There are some photos of caterpillars in the image gallery on the Butterflies and Moths of North America web site. If you have a photograph, you can submit it to the site for help with identification by a regional expert.
For step-by-step identification assistance, try the Caterpillar Guide of the IDnature guides series.
The Owlet Caterpillars of Eastern North America is an excellent reference for this region and species group.