Wednesday I took this photograph
When I was photographing this guy I noticed that he seemed smaller than other Monarchs that I had photographed. So when I got home I checked the Queen which is similar, and quickly determined that it wasn't a Queen and didn't bother to look further. This afternoon I was reading other birding blogs and Susan Gets Native has a photograph of a Viceroy.
I immediately got the books out, and they all say the Viceroy has "a black line usually curving across the hind (lower) wing". Well, this guy only has a hint of the black line. But on a closer examination of the depictions in my books there is an obvious difference in the fore (upper) wing. Additionally, there is a difference in the number of white spots around the eye, as well as coloration on the abdomen. The Viceroy is 2 5/8 to 3 inches and a Monarch is 3 1/2- 4 inches. A rather significant size difference.
For comparison sake, here is a Monarch that I took earlier this year.