How do you test for thermal leaks in double paned windows

A couple of suggestions. If the seal is broken for the double pane window it will often build up condensation between the double panes during cold times(winter). The condensation will generally stay there-depends. A thermal imaging camera can generally tell where these kind of problems exist. This works best when there is a significant difference between inside and outside temperatures. The idea is to look at your home and see what the temperatures are and compare the window suspected with the others and see if there is a significant temp difference. Again, winter temperature differences show a better picture. I have and app for my IPAD that has a thermal imaging component, but probably not good enough for what you are asking. A glass company probably has a way to detect, but I have never seen them do anything, so I don't know their methods. I know that condensation between the panes is a definite sign the seal has been broken. I am not a glass expert, just had windows and sliding doors where this has happened to me and replaced them.

An excellent way to check any type of thermal leak is with a calibrated infra red camera. They can be rented at many rental supply houses for a nominal fee. The best time to do such an inspection is as studs ad said when there is a notable difference in temp between indoors and outdoors. You will also likely find several other areas that need minor repair to fix thermal leaks. If it feels cold near a window in the winter 9 times out of 10 it's the sealing around the edge of the window rather than a faulty thermal pane.