Don't wait too long to see if it's a false alarm. Treat an environmental emergency in the cockpit as just that; an emergency. Carbon monoxide has a tendency to remain with a red blood cell 240 times longer than oxygen does, or more accurately put, the red blood cell has a greater affinity for CO than O2.
What this means for you is that while you can recover quickly from an oxygen-deprived state such as hypoxia, by getting good ventilation, oxygen, and pressurized air...the same isn't true of carbon monoxide poisoning. It's a slow recovery, and you may not have the time. Even with 100% oxygen in a pressurized mask delivery, it may not help. Point is, take any indication of carbon monoxide in the cockpit very seriously.
The most common source of CO in the cockpit, of course, is engine exhaust. In automobiles, cabin heat is obtained by blowing air over small radiator full of circulating engine coolant. Not so in most light airplanes. We use shrouds over the exhaust through which outside air is funneled, warmed, and put into the cabin. These shrouds are notorious for slippage, often held in place with nothing more than PK sheet metal screws, hose clamps, or both. They tend to slip out of position, admitting any leaked exhaust gas, and the tend to slip and cause wear on the support cones beneath...leading to exhaust in the cockpit. You may not smell it, and certainly you won't smell CO (carbon monoxide)...but it may be lethal all the same.
Assume that any indication you have of carbon monoxide in the cockpit is real. Ventilate as required, land as soon as practicable. You can always sort the matter out on the ground. If you wait on the assumption that it's probably not a big deal, you may reach the ground in an unintended and undesirable manner.