How can I check for possible damage after water leaking into loft chimney?
The best way I've found to check for damage isn't the way you want to hear: pull everything out, open it all up, and expose any areas that could have been damaged.  Faced with that, most of us ignore the problem until there is a visible sign (rotting wood or water damaged drywall).  But that's not to say this is the right choice, since mold and mildew will spread and make the problem worse.

If the problem is behind a few sheets of drywall, the best option may be to simply pull them down and replace them with a mold and mildew proof board.  Do a thorough inspection for damage while the drywall is out.

The probe sounds like a wood moister tester.  A high reading is bad, but a low reading doesn't mean you're ok (the water could have gone somewhere else).  If you can't access the affected area from the attic or crawl space, then you may want to see if you can rent a borescope, drill some holes near where you think the water could have gotten in, and see what you can find.