In order to know if a door or window is still in use, you stick some ABC (Already Been Chewed) gum right where the door meets the frame and then let dry. If that door or window is still in use, the seal will be broken. A good way of finding out if a building is still in use while scouting.
Also, if you're into ceramics or just happen to have clay available(the non-modeling kind that dries)it's very useful for this. It drys quick and has a clean break, in addition to not being super sticky.