Assuming a compressor has a reasonably good filter system on it, the value of one-time air testing is somewhat questionable.
To be truely useful, air testing must be done regularly and repeatedly, so a pattern emerges. A single air test gives you a snapshot of the air quality at that moment in time. If you do it just after getting your compressor up and running, it will probably tell you the air is good, no surprise, since the filter cartridge or media will be brand new. But it will tell you nothing about how the air will be after 20 or 30 hours of operation, when the media is starting to age. Most small compressor owners would probably do better to install a moisture monitor and make a point of changing their filters a little more often, than spending money on a single air test.
In the case of an IC-powered machine it can be of more use, since CO is the big danger, and an air test can troubleshoot and warn of a bad installation. But CO levels can change radically on the same installation, depending on wind direction, whether a door or window is open or not, or other trivial changes, so once again, a single air test is no guarantee of anything. So for, buying a CO Experts monitor and adapting it to your compressor would probably buy more safety for the dollar than a one-shot air test.