I see people having difficulty telling an American Lady and a Painted Lady butterfly apart, so I have compiled this handy chart to refer to and make it easier with which to spot the chief differences between the two at a glance.

On the left side of the image, an American Lady, open-winged and closed-wing view. On the right, a Painted Lady. The chief difference, the one that is easiest to identify for most, is the tiny white spot on the American Lady. It is located in the postmedian area of the forewing. In the corresponding area of the Painted Lady, no such spot will be found.
Another good identifying mark will be the eyespots in the postmedian area of the hindwings of both butterflies. On the upperside, they are quite similar in size for both. But on the underside, two large eyespots can be clearly seen on the American Lady, whereas four are discernible on the Painted Lady and they are much smaller.
There are other small identifiable features unique to both species, but at a glance, these marks I've described should be of great help for most people. For further identifiable features, see here.

American Lady, top left - taken on June 12th, 2007.
American Lady, bottom left - taken on June 21st, 2007.
Painted Lady, top right - taken on October 29th, 2007.
Painted Lady, bottom right - taken on October 9th, 2007.