Look inside at your metal chimney and locate the connection pipes. If there is smoke is coming from the pipes when your furnace is on, then it should be cleaned.
Use a step stool, if necessary, to view the top of your metal furnace and check for any signs of soot. The more buildup of soot on your furnace indicates how dirty the inside of the chimney is.
Examine the hot water heater portion of the unit and also look for another sign of a buildup of soot.
Step near the furnace to see if you can feel strong heat emitting from the unit. Extreme amounts of heat are signs the chimney needs to be cleaned.
Use a carbon monoxide detector to detect high amounts of carbon dioxide near your chimney and in your home. If the detector goes off, then you should turn off your furnace immediately and contact your local gas and electric company.