Support the sterns of the platform and then lift the one bow. The difference between the height of the two bows above their supports is then measured. Best way in my opinion would be to lift both bow clear of their support by picking up one bow, then lower the bow you are holding too such an extent that the lower bow just touches its support again. This will give a measured of the vertical platform stiffness

Horizontal stiffness is measured by pulling on the bridle wires and see how many the bows are pulled inward by the bridle loads.

Wouter