So what did old Stradivarius do? I imagine he shaped the tops and checked for resonance all the time until the top gave off the sound he wanted. He would have used a small leather mallot to tap the wood.
placing sand on the soundboard and running a bow along the edge of the soundboard will set up a vibration that will reaveal a specific pattern in the sand.
He may have also used this technique to achieve a particular resonance