Boatgerl,
Between my wife’s sailboat and my vintage C-Dory, we’ve gotten fairly familiar with how to use moisture-meters to assess the condition of cored fiberglass. That said, we're not hot-shots; we've just been around the block a time or two... Dr. Bob (aka, thataway) has just a little bit more experience with boats and boating than I do. I agree with him, that tapping a hull is an important skill/method to learn to assess the condition of your boat’s fiberglass cores. Another 2¢ of mine is that I’d go over all cored areas of the boat with the pinless moisture meter, marking the location of notable changes in the readings (say, 10%, 20%, 30%) with a grease pencil - basically making contour lines of those variances. Then, I’d get out the hammer of your choice to double-check and refine the characterization of those readings. Lots of info on the web on that as well.
I don’t know that I’ve written the moisture meter approach very well. If you’re interested in chatting about how-to, PM me and the wife and I will be happy to help out.