no responses yet...Ok, besides a surveyor with specialized moisture meters and little hammers, you could try doing some soundings yourself with a light polyurethane hammer, first finding a solid area and compare sounds.
I would take a million CP spotlight and look over every inch of hidden space with it. Then I would take an ice pick and do a little probing. Look under the carpet on the floor for wet/damp areas; if you find them there's a chance that the stringers are wet. Try using your weight to see if the floor gives.
Transoms are a little harder to spot moisture damage but not at all impossible. Sometimes you can simply press with your hands and find damp areas. Look closely around any and all through-bolts.
IMHO, dry rot is a misnomer; you're looking for damp rot actually. Good luck.
Buzz