If I Can Stick My Finger Through It,
It's Probably Rotten
I have done dry rot repair throughout California and have used many methods to detect and find dry rot and water damage in buildings. One of the best methods in getting an early indication of the amount of rotted wood that may be in sight may be poking around with a finger. Wood soft enough to crush with such a small amount of force would require some further investigation.
Do you have areas where the wood is soft enough that you can find it by hand? You may need to contact a contractor in your area with experience in the detecting and removal of dry rot and water damaged wood.