So, we all use little hygrometers in our 'dors. They are run with a little electrical cell that varies according to the RH. But, in a lab, you use a more fundamental measurement instrument called a wet bulb/dry bulb hygrometer. Basically, one of the thermometers is kept in a wet sleeve. This is cooled by the evaporation. This difference between the two can be converted to RH.
This is the one we built for the lab where I work. And, no salt test is needed!
