”what has to be removed for inspection.”
I wish I had an easy answer for you on this one, unfortunately I do not. We discovered the problem when we had obvious core failure, so our “inspection” was peeling off the upper deck surface until we go to sound core. Miles’ approach of pulling screws from the hatch frame and looking is I think as good as it gets.
There is a preventative that should be considered. Remove the hatch and drill out the screw holes in the deck significantly larger than the diameter of the screw. Drill down all the way through the wood piece. Fill it with epoxy thickened with LOTS of high strength thickener. Redrill a pilot hole for the screws, and again drill all the way through. You want isolate the leak path from the wood, and you want to SEE a drip right away, not have it get trapped. If you go this route, be prepared for surprises. Not all the screws, and I bet not all the boats, are fixed in the same way!
Doing this is a bit fussy, requiring a bit of disassembly of the interior trim and for the hinge side screws it has to be done very carefully or else the screws will pull out in short order. There are some other viable alternatives for resetting the screws, but pretty much all of them require pulling the hatch.