The cheap (£5-£15) damp meters have sharp pins that you poke into the wood, making holes/scratches, otherwise they measure damp on the surface, that either you can see and feel, or got there because of simple condensation. (resistance measurement)
tbh, I wouldn't waste money buying one.
To do a reliable non-damaging test you need the contact less type, that cost North of £200, these measure moisture within the bulk of the material, and can ignore surface moisture. They should find where damp is getting into the insulation as the inner panels are quite thin.