Aside from the obvious cracks or cracked glass it can be hard to tell if your windows are working as they should be. Here's a listing to check when inspecting your own windows. If your windows have one or more of those issues then it is time to call a professional.
Won't budge: if your window is quite tough to start or if it doesn't open at all then you have a issue with your frame. If it is wood then the frame may have enlarged or rotted. Alternatively, the metal on the framework could be rusted or a mechanism at the window is broken.
Drafts: if you can feel a draft whenever you're near a closed window then there's something very wrong with your window. If you realize that a room is hard to keep warm (or cold) then you may want to try out the candle trick. Turn off any heating or cooling so that there's hardly any air movement. Then light a candle near the window in question and move it around the frame slowly. If you see the fire flicker then you have found your draft. There is a crack somewhere in the framework or at the glass that allows moisture to enter -- severely reducing the insulating material of this window.
Moisture between panes: If you have moisture in between the panes of a double or triple pane window, then the window has been endangered. There is a crack somewhere in the framework or at the glass that allows moisture to enter -- seriously reducing the insulating material of the window.
