Eastern tiger swallowtail butterflies are one of the most recognizable species of butterfly in North America. Their bright yellow and black wings resemble those of the monarch butterfly, but the blue spots and pointed tail projections on the wings of the Eastern tiger swallowtail set them apart. Unlike most butterflies, female Eastern tiger swallowtails have two color phases. The females can be almost identical to the males, or nearly solid black. Black phase females have the same banding pattern as the males on the underside of their front wings, but the pattern is faint due to the dark color.
Position yourself so you can view the butterfly from the top.
Examine the main color of the wings. If the butterfly's wings are black, it's a female. If they are black and yellow, it can be male or female.
Look at the blue spots near the rear edge of the wings. Males have fewer and smaller spots than females. Sometimes they only have one or two small spots near the tail. The female has larger blue spots that are arranged in a broad row.