The interesting thing about CO alarms is that they only activate once the ppm reading hits 100 , which according to my gas training course , symptoms may already have started to appear(35-50 ppm is considered unsafe for prolonged exposure) so vigilance is required.
If you feel odd then something may be up, Generally if CO is being produced a lot of CO2 will be produced and you can smell that . I use the ones with a digital display so I can see the maximum value it reached . I also have a digital room CO2 detector, which does fluctuate wildly even in my front room, so not convinced its accurate( We used to use CO2 detectors which used thermal resistance to control CO2 in our range of specialist incubators and their accuracy was heavily dependant on the relative humidity) Infrared absorption sensors are more accurate ( used in Car Exhaust and flue gas analysers) but are a lot more expensive. There are several CO2 monitors on the market at reasonable prices , but they don't state what technology they use and as most are made by our chinese friends it difficult to assess their reliability. And finally smoke detectors ,whether optical or ionisation ,don't detect smoke, they detect small particles in the air ( one of which is smoke) and the test button doesn't test the detector, just the buzzer. I have a 'can of smoke' and another of CO for fully testing the detectors ( the CO detector has to be put in a plastic bag and the CO injected in to test it to get it upto 100 ppm. As the fire service advise, any detector ( with a good battery) is better than no detector