Hi, Cindy. drywall or drywall is made from gypsum. Water bonds to gypsum by what is called "waters of hydration". In the manufacturing process for drywall, these "waters of hydration" are driven off by heat. If your drywall gets wet, the gypsum attaches to water and it expands. The only way to get rid of this attached water is to heat the drywall under controlled conditions in an oven.
I don't know of an instrument outside the laboratory that will check for the water except your eyes and your fingers. In the lab, you can heat/dry a sample and measure the amount of water given off. In the real world, expansion if the drywall would be visible on the surface and the product would feel more spongy than drywall that has never been wet.
You probably can look of a web site for somebody like U. S. Gypsum for more on this. You might try e-mailing them to see if they know of such an instrument.