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 by Amy Bartlett Wright and Caterpillars of Eastern North America : A Guide to Identification and Natural History by David Wagner.