Fogging or condensation between the panes of glass: If you find either of these things happening in your windows, it means the seals have failed and moisture is getting between the window panes
You have a difficult time opening or closing your windows: Aside from the fact that this can be a fire hazard, it also means your windows may be imbalanced due to improperly functioning mechanisms
Drafts: Drafts are concerning because it means the window is faulty or outdated. It also means that warm or cold air is getting in and out of your home, driving up heating and cooling costs.
Trimwork or wallboard around the windows are wet: the elements are finding a way in around your windows and compromising the frameworks integrity and potentially compromising the interior and exterior of your home.