There isn’t a self-diagnosis option for carbon monoxide poisoning, but anyone with confusion or a loss of consciousness should have 911 called for them.
If carbon monoxide poisoning is suspected, all occupants of a building should go outside to breathe fresh air, along with calling 911. If you suspect CO poisoning, don’t try to drive; call an ambulance.
You should suspect carbon monoxide poisoning if more than one person in a building with a combustion source (furnace, fireplace, gas appliances, wood-burning stove, etc.) is complaining of headaches and nausea.