Carbon Monoxide

Before getting too wild, get a monitor or detector. You can buy very simple $4-5 CO detector that will change color in the presence of CO; you can get these at many pilot supply stores. (A CO problem is always a possible issue in small aircraft as the cabin heater is a 'stove' around the exhaust and any leak there will push exhaust into the cabin.) Just put one on the instrument cluster near you and watch.

Look here:
http://www.aircraftspruce.com/menus/ps/carbonmonoxidedetectors.html