How can I tell if my double pane windows are bad?

While double pane windows are excellent for thermal efficiency, they are not flawless. Some double pane windows may defect over time, usually along the seal. Clouding or fogging in between the windowpanes is the first sign of a failed seal. If the window seems dirty even after cleaning, the problem could be between the panes. If there are visible drops on the window, and they are not due to moisture outside or inside, your double pane window has a broken seal.  

Obviously, any crack in the window is also a sign of window failure. A window crack will quickly lead to a foggy window and failed thermal efficiency. Double pane windows will not always fog if the seal is broken. Sometimes, they simply compromise their thermal efficiency. On a particularly cold day, compare the temperature between a good window and one that you suspect has gone bad. Place your hand alternately on the inside of the good window, and then the bad. If there is a noticeable difference, you may need to replace the window.