There are a couple of different ways. Hook up a vacumm guage and then take something like a soda straw and stick it in the vent hole which sits above the piston. Flick the throttle while watching the guage and the straw will rise out of the hole when the spring overcomes the lowered vacumm. Not very accurate though. The way I'm doing it is I have a spare quadrajet (which is dismantled), so I can hook it up along with a guage. There's a small vacumm hole directly below the piston. Then I can start the engine and slowly pinch off the hose going to the manifold vacumm port. The guage will slowly drop and when the vacumm gets to the corresponding spring pressure you can see the power piston slowly start to rise.
Fun stuff