If you find that double-pane windows in your house appear foggy or misty, this likely means that the silica desiccant that is keeping moisture from entering the window has failed. The window will need to be repaired or replaced.
Double-pane windows have a layer of gas in-between the panes to insulate it. This air is sealed off to keep any moisture from entering.
As the sunlight heats up the gas in between the panes, the gas expands, and then, when it cools off during the night it contracts, thus causing slight movement of the window panes that will slowly cause the seal to fail. Once the silica desiccant has been saturated, condensation will form between the window panes.
This will be a more common problem on the sunny side of the home since there are greater variations in temperature there.
You will not always notice condensation, but using a thermal imaging tool, you can note differences in temperature that will indicate that the seal has failed. There are trained professionals that will be able to detect any problems.