Most surveyors carry a moisture meter such as the Tramex Skipper. It doesn't pierce anything. You press it against the gelcoat and the reading rises with the moisture in the material under it.
The reading is a comparison. Tramex's guide says to compare readings below the waterline with dry areas above it, and Practical Sailor's tests found meters can't give a true moisture percentage in fibreglass. A high reading near a fitting means look harder there.
How a surveyor uses it
On a dry, clean surface, in a grid, against a dry reference area on the same boat. Below the waterline after the haul-out, once the hull has dried. On the deck round every fitting, and on the transom round the engine mounts. Then the hammer to confirm: a wet area that also sounds dull points to wet core or delamination.
Some antifoulings contain metal and read high, so Tramex suggests removing it on that spot and testing again.
In the report
Name the meter and its scale, then list the areas that read high against the dry reference. Log readings with our moisture readings log.