buy an inspection camera from Harbor Freight tool ($89) or Costco ($119).
Plug the snake head of the camera/lighting thru the holes around the suspected wet area (you drill those holes).
If you can borrow a thermal image camera ($2500) from anyone, a quick scan can tell you th elimit of the damaged area without tearing the dry wall.
seal the exterior water inflow is a must before oyu do anything.
