Examine the size of the butterfly. Brushfoots and swallowtails are typically smaller butterflies, while admirals, emperors and longwings are typically larger.
Look at the colors of the butterfly's wings. The Pieridae family of butterflies are typically white and yellow. The Lycaenidae family of butterflies may be blue, copper or purple. Use the color clues to determine what family of butterflies that it belongs to.
Consider the surroundings of the butterfly. Brushfoots and milkweed butterflies are commonly found in wooded areas. Monarchs, emperors and snouts typically perch on dead leaves, and these can be found in the forest or places that humans frequent.
Look at the shape of the butterfly. Monarchs have long wings that are shaped like a lima bean. Swallowtails have very wide wings that look like the wings of a fighter jet. Brushfoots have rounded wings that make the butterfly's body look like a big triangle. Admirals have wings that come to a point at the top.
Use a field guide or Internet resource to look up the exact species. A number of web resources provide histories and pictures of butterflies, such that you can match the butterfly with its picture.