drywall doesn't dryrot that I am aware of. It is not a wood product. As Henry said it is gypsum and damage to it is usually water which softens it. The only testing device I use and have seen used around drywall is an electronic studfinder. It basically tests the density of the wall and since the studs behind the drywall is much denser than open space, it finds them. If drywall got wet or spongy, then replace it. If your home flooded and there may be water inside the studwall, then remove some drywall or drill some holes to look for the damage.
Post back with more info on what damage and to what has occurred and what you are looking for. If some insurance adjuster did this then it is highly suspect.