A smoke pencil is typically a hand-held device that emits a chemical smoke when squeezed. Smoke pencils can be used to quickly and accurately determine the air pressure relationship between two spaces--for example, between the indoors and outdoors or between a bedroom and the space inside the wall separating the bedroom from the hallway. If smoke gets sucked into a crack or opening, the smoke pencil is located in a region of positive air pressure with respect to the crack.