Can you tell a moth caterpillar from a butterfly caterpillar?

There is no feature that will distinguish a butterfly caterpillar from a moth caterpillar. You can only tell them apart by learning what the caterpillars of each butterfly and moth group look like.

There are books that can help you learn about caterpillars, such as the Peterson's First Guide to Caterpillars (Amy Bartlett Wright), Caterpillars of Eastern North America : A Guide to Identification and Natural History (David Wagner), Caterpillars (Chris Earley), and Owlet Caterpillars of Eastern North America (David L. Wagner, Dale F. Schweitzer, J. Bolling Sullivan, and Richard C. Reardon).

The image gallery at Butterflies and Moths of North America contains a lot of excellent caterpillar images.