These days, many smoke detectors have built in CO sensors and there are stand-alone plug-in models available at most department and hardware stores.
If you know anyone in the fire Dept., the gas company, a home inspector or that works in a plant that handles chemicals, they most likely have a portable gas meter that will do the trick.
Myself (falling in the latter category) I've checked my house out and most people would be shocked how much CO fills the kitchen everytime you turn on your oven!
Dave