This is the one I use
Moisture Meter.
It comes with calibration stock materials, and tells you % moisture, with a damp, moist, wet, settings. I have yet to have it give me a false reading, although it has some limitations. You can't use it through metal based bottom paints, and it will peg on ferrous materials inside the glass.
If anyone wants to check their boats out at Longpoint in the spring, I plan on being there, and will bring it with me. Just remember, it and I are only the messenger. I have had some people get upset, especially when I am surveying a boat for sale when I show them wet transoms, coring, and stringers.
Here is a good article on interpeting moisture meter readings
article .