Well, whatever. Highspeed machines would benefit from laminated magnets.
Anyhow as I said before you need all of the magnets and one test coil, and then take 95% of that to get open circuit volts.
Alternatively you could setup.. a single magnet, or three of them. Fix the rotor to a pendulum and let it swing back and forth ( for 40 magnets) 18? degrees of travel. I suppose the volts you get out might be accurate... idk. Anyhow it will be rather low frequency so you'll have to rectify the output, charge up a capacitor and measure the dc volts with a high impedance volt meter.of course this assumes zero lost volts in the diodes
Anyhow I can't see how you can measure the volts from a single coil and a few magnets without an oscope and a spectrum analyzer.. if you do the pendulum gig, an oscope alone should suffice