It's a shame you guys don't have access to the Wikipedia, so I'll lend you my copy:
"The gas sensors in CO alarms have a limited and indeterminable life span, typically two to five years. The test button on a CO alarm only tests the battery and circuitry, not the sensor. CO alarms should be tested with an external source of calibrated test gas, as recommended by the latest version of NFPA 720. Alarms over five years old should be replaced but they should be checked on installation and at least annually during the manufacturers warranty period."
The alarm points on carbon monoxide detectors are not a simple alarm level (as in smoke detectors) but are a concentration-time function
. At lower concentrations (e.g. 100 parts per million) the detector will not sound an alarm for many tens of minutes. At 400 parts per million (PPM), the alarm will sound within a few minutes. This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to relatively common sources of carbon monoxide such as cigarette smoke."
There are UL and other standards, perhaps there is a USCG or ABYC standard that makes "marine" detectors different in terms of what set points they'll use.
But just TRY to find a spray can of CO, or one of smoke, to test your detectors with. Won't be at the local bigbox store, for sure.