Moths are identified based on the time of year they are found; where they are found; their general shape at rest; the color, pattern, size, and shape of their wings; and sometimes by the food that they eat. They are usually most active at night, unlike butterflies.
Adult moths can be identified as separate from butterflies based on their feathery or thick antennae and their horizontally laid wings as opposed to the curled antennae and vertical wings of butterflies. Once the moth is identified, the species can be determined based on its general shape and wing patterns.
Discoverlife.org provides a checklist to help narrow down the possible species of moths. About 165,000 species of moths have been identified worldwide.