The easiest way to tell if the fireplace flue is open is by placing a hand into a cooled fireplace and feeling for cold air. If there is a burst of cold air, the flue is open. Ceiling-to-floor metal fireplaces have a metal latch located on either side of the fireplace that allows for easy testing of the flue.
Another sure-fire way to tell if the flue is open or closed is if smoke comes billowing out into the room once you have lit a fire in the fireplace. If this occurs, it is important to open the flue immediately as well as any nearby windows to guide the dangerous smoke out of the home as quickly as possible. Inhalation of fireplace smoke can cause coughing and breathing difficulties, and fireplace soot can leave stains on the walls and carpeting.
It is always important to close the flue once a fire is completely extinguished in the fireplace, because chimney openings are large enough for small animals such as possums, squirrels or baby raccoons to pass through and eventually enter the home. Additionally, birds are known to set up nests in chimneys, and they can easily fall or fly down the chimney shaft.