Here's a Spot-winged Glider, happily perched in Glider Glade.
Do you see the small dark spot on the hindwing? There's another even smaller dark spot in front of the main spot, but the main spot is the one you look for in the field. Here's a little closer look at this individual.
A view from above shows the spots a bit better. The next photo shows the main two dark spots. They are so close to the abdomen, however, that they can be hard to see. The one in the hindwing closest to us is an example of how the spot can almost disappear against the abdomen.
Spot-winged Gliders are uncommon in the Pacific Northwest, but Betsy and I did see one and photograph it at Magnuson Park in July 2012. Here are the relevant links: