Limiting this response, checkin air fills for nitrox content, I do not. At some level, one has to function on a little trust in those you do business with. (I do test for CO if source is other than my LDS). My LDS is where i get "local air" an I am one of the fillers. I know all their nitrox is in clearly labeled tanks, and have faith in their system of filling, storage and labeling. When on air on a dive boat, I assume that air is air, not some other mix. On liveaboards, I do the same, and when renting tanks from a shore concession, I also believe them as to air fills. So far so good. I read in this thread for the first time someone getting nitrox when they thought they had air. So far, Debbie and I have been safe. With a combined use of about 1700 air fills, I like our percentges.
DivemasterDennis