The Viceroy looks very much like a Monarch. There are several differences, but the two most immediately obvious are size and a a black postmedian band across the hindwing. The wingspan of a Viceroy is 2 5/8 to 3 inches, and a Monarch is 3 1/2 to 4 inches. To translate what a black postmedian band is, it's a black stripe on the lower wing. This black stripe can be faint or lacking.