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Gurk, there are "moisture meters" sold to check for internal moisture in wood, concrete slabs, and cored fiberglass hulls. Each type has a slightly different range and probe tips but that's what a professional surveyor uses to determine moisture in a hull/deck. Probably $300-400 to buy one, which is why using a hard rubber tap hammer is still so common. Walking BAREFOOT on a deck, you can often feel a change in the springiness if there's a problem underfoot. Bare hands also work (have you eer tried to locate a stud in a plasterboard wall? but on a large deck, the tap hammer can save some raw fingertips.
The folks at West System Epoxy have a lot of free technical material available on their web site, and they are also generous about answering question on the phone, regarding what materials, processes, etc. you need to do proper repairs. They make good stuff, too.