The PNG Test
The PNG test is easy, really. All it is is putting a PNG on a dark green background. (I like to use something close to #003300. Why dark green? Because some images have a black border.) If there are no traces of the image background leftover, the PNG passes!
On the left is a PNG (extracted by yours truly) that would pass the test. On the right is a random one I found that would not. Click the image for a bigger picture.