One possible solution consist in the use of a transparent water tank like the aquarium, a stock and pulley with string.
A set of measuring points before and after tilt will allow to detect the tilting angle and position even when the boat will move inside the tank, the angle will be keept constant anyhow.
Crossing the angle line with the vertical position will provide the “pivot” point .
ClaudioD
Or you could just hold the boat horizontally over the surface of the water. Slowly lower it down until the hull touches the water. Continue to lower it and see where the boat pivots around. Does it pivot around the CG, lifting the hull up out of the water? Or does the hull stay stationary on the surface with the keel rotating down below it?
I have found many references that imply that the boat rotates around the Center of Buoyancy, including Marchaj’s Aerohydrodynamics of Sailing as pictured below. They assume that you already know that it does. The only explicit mention is from a book called The Modern Cruising Sailboat by Charles Doane.