The salt test checks a hygrometer against a known humidity. A saturated slurry of table salt and a little water holds the air above it at about 75% relative humidity, and that figure changes very little at normal room temperatures. Seal a capful of damp salt and the hygrometer in a bag, wait several hours or overnight, and read it without opening the bag. The difference from 75 is the hygrometer's error. Calibration kits with a 75% humidity pack do the same job with less mess.
Also called: hygrometer calibration