Open the door to the fireplace or remove any grills that may surround it. Examine the exterior of the fireplace for a lever, or a round hole with a rope or chain emerging from it. If you don’t see one, look inside the front edge of the fireplace box for a lever.
Grasp the rope or wire and pull it out as far as possible toward you to open the damper flap and clear the flue. If you have a lever inside the box, pull it toward you and then lower it down to engage the notch that holds the damper open.
Kneel down and peer inside the top of the fireplace with a flashlight. When the flue is open, you can see straight up into it all the way through the chimney. If the flue is shut, you will see the metal flap of the damper still over the bottom of it.