Composite showing, on the left, American Painted Lady, Vanessa virginensis, and on the right, Painted Lady (Cosmopolitan), Vanessa cardui. The two are easy to tell apart from below, but a bit more difficult from above. Here are is a list of what I think are significant marks:
a-V. virginiensis has a larger eyespot here. In V. cardui all the eyespots are about the same size.
b-V. virginiensis has no bar connecting the two large spots in the median area. V. cardui has a heavy connecting bar. This is mentioned as the key mark by Glassberg (1).
c-This bar is usually longer in V. virginiensis, almost reaching the leading edge of the wing.
d-V. virginiensis sometimes has a white spot in the orange area here, while V. cardui does not. This character is variable, however. Both of my good photos of V. virginiensis from above lack the white spot. (It does not rest with wings open very often, so I have few dorsal photos.) Glassberg (1) mentions this as a good character, but it is barely visible in the photo he uses.
e-The shape of the forewing is, I believe, a consistent character for separation. V. virginiensis has a prominent notch in the forewing, so it is slightly falcate. The forewing of V. cardui is more rounded. Though not mentioned in the field guides, this character is consistent in the images I have examined on the Internet and in print references.

A composite from these two images, both from Durham County, North Carolina, in September: