ADT home security setup comes with a CO detector that calls the fire department automatically if high levels are detected.

I pay $52/month for ADT, which gives me burglary, fire, and CO protection.

A cigarette will work too, according to this site:

How To Test a Carbon Monoxide Detector

(Don't some of those warnings on cigarette packs say the smoke contains carbon monoxide?)

And there are also test kits that contain vials of CO with a plastic bag that you can put your detector into to check them. If you have a detector that has a digital readout of the levels then the cigarette test should work fine, but if it's just one of those combo smoke alarms then the levels from a cig may not be high enough to set it off. The firemen that were here said mine with the readout was definitely the one to have.

[EDIT]
This is the one I have BTW:

Kidde CO Detctor at Amazon.com

Anyone know how to test to see if they actually work?

Someone said light a match and then quickly put it out next to the sensor to see. Any other ideas?

Just a reminder to everyone how important a CO detector is. Mine just went off, I reset it, it went off again, so I called 911. Elevated levels throughout the whole building. Please don't think it can't happen to you. We now have to get a chimney person out here to check things out, until then no hot water. BOO!! But everyone is safe thankfully. Please, let's not see another story on the news about deaths from CO poisoning.

My uncle, aunt and cousin nearly all died of CO. Thankfully my uncle is a big guy and stayed concious long enough to dial 911 (at this point my aunt and cousin were already out). No detector in the house. Make sure you get one, kids!

CO monitor works like its supposed to.. Thanks to it was able to fix a broken a boiler hose that brought the fumes into the apt.

Yes, very important. My CO detector saved my life a year ago Christmas night.

Did you know that CO poisoning is the leading cause of accidental poisoning deaths in America? If you don't have a working CO detector (only 27% of Americans do), get one! Here are some interesting, often overlooked facts about CO dangers:

* CO is a produced anytime a fuel is burned. Potential sources include gas or oil furnaces, water heaters, space heaters, clothes dryers, barbecue grills, fireplaces, wood-burning stoves, gas ovens, generators, and car exhaust fumes.
o CO poisoning is the leading cause of accidental poisoning deaths in America. (Centers for Disease Control)
* Every year more than 10,000 people die or seek medical attention due to CO poisoning from home-related products. (Consumer Product Safety Commission)
* More than two-thirds of Americans use gas, wood, kerosene or another fuel as their home's major heat source.
* 65% of CO poisoning deaths from consumer products are due to heating systems.
* Only 27% of homes in America have carbon monoxide alarms, according to the Hardware/Homecenter Research Industry.
o An idling vehicle in an attached garage, even with the garage door opened, can produce concentrated amounts of CO that can enter your home through the garage door or nearby windows.
o CO poisoning deaths from portable generators have doubled for the past two years, and many of these deaths occurred in the winter months and during power outages.
o A poorly maintained gas stove can give off twice the amount of CO than one in good working order.
Prevention
o Install at least one battery-powered CO alarm or AC-powered unit with battery backup on each level of your home and near sleeping areas.
o Do not use charcoal or gas grills inside or operate outdoors near a window where CO fumes could seep in through a window.
o Check all carbon monoxide alarms in your home. Do they use the most accurate sensing technology? Do they need new batteries?
o Replace CO alarms every five to seven years in order to benefit from the latest technology upgrades.
o Have a licensed professional inspect heating systems and other fuel-burning appliances annually.