Yesterday I was with my wife and three children in the car, 300 KM from home just after sunset. I pulled into a gas station and as before all long trips, I dialed 31 PSI on the gas station's air compressor and attached it to the tire. The air compressor was inoperable, and let out enough air to bring the tire down to 26 PSI before I realized which way the air was flowing.
How might I have tested the compressor beforehand?
Simply pushing the "flat inflate" button would not be a good test, as in fact this machine had a few PSI residual and the "flat inflate" button did put out spurts of air. Running "flat inflate" until the compressor comes on won't help as I cannot be certain that 30+ PSI at the sensor translates to 30+ PSI at the outlet (I've had this problem in the past, too).