Carbon Monoxide (CO) is invisible and odourless, so you can't actually "watch out for it". However, you should monitor for it because an excessive build-up of this gas in your home can be deadly.
Fortunately, there are many types of Carbon Monoxide detectors you can purchase — and most are effective and affordable. Some models simply plug into an outlet. (Many also have a battery backup.) 
Carbon Monoxide is caused by the incomplete burning of fuel. It can be released by a faulty gas furnace, kerosene heaters, and gas fireplaces. That's why it's a good idea to install detectors in areas close to these fixtures, as well as near bedrooms.
Experts say you should always follow manufacturer's instructions when installing CO detectors, and test them regularly. You want to make sure you can hear the alarm from your bedroom. CO build-up in homes is rare. So your detectors may never go off. But, if the alarm does sound, get everyone (including pets) out of the home and into the fresh air. Then call 911. Typically, the fire department will do an inspection and determine the source of the carbon monoxide.
A final tip: Never use your BBQ or outside grill in the garage or, especially, anywhere inside your home. The risk of CO exposure is very high and definitely not worth the convenience of a grilled burger!