I haven't looked at my DPF yet, but when this awful weather improves I might crawl under my van with a camera and take some pics. In the mean time, a google image search for nissan diesel particulate filter should give you an idea of what you are looking for- bulge in the exhaust pipe, fairly close to the engine, with small-bore pipes coming out of it for pressure sensor etc.
Not sure if CO2 figure would reveal much. I thought the DPF was a mechanical filter designed to trap solid particals. CO2, being a gas, would pass straight though? (I'm no expert though, and would gladly be corrected on this)