When it comes to Carbon Monoxide, it is impossible to know if you have a leak without CO detectors. If you have any gas burning appliances, an alarm should be installed on every level of the home and in sleeping areas.
Initial symptoms are similar to the flu without a fever and can include dizziness, severe headaches, nausea, sleepiness, fatigue/weakness and disorientation/confusion.
Any fuel-burning appliance that is malfunctioning or improperly installed. (Furnaces, gas range/stove, gas clothes dryer, water heater, portable fuel-burning space heaters, fireplaces, generators, wood burning stoves, vehicles, blocked chimney, and bbq grills used in enclosed spaces.)