The easiest way to tell is just by looking at them, if your windows have mold around the frames or moisture fogging up inside the glass they have likely failed. If your home was built before 1991 you most likely have aluminum windows which are terribly inefficient. High energy bills and cold, drafty rooms are obvious indicators, but there may be more subtle signs as well. A window that doesn’t close or lock easily may have a compromised frame which can lead to structural damage and dry rot where you can’t see.
