Doing this sort of measurement reliably and accurately enough to trust strikes me as very hard, unless you can take an existing solution. Presumably the people you're testing are expected to be able to find out what flow rate they're set up. This suggests that there may already be some flow monitoring equipment in use. Do you know what it is/how it works? Perhaps it would be possible to either duplicate that, or piggy-back some sort of monitoring on top of it. Otherwise the only sensible approach I can think of would be to put a small, known restriction in the supply and measure the pressure difference across it. Doing that safely when there is pressurised air involved may not be easy.