If you have an ohm meter and know how to use it. Check the resistance across the white and green wire at the Vreg (disconnected)
Next disconnect the rectifier and check the resistance of the Yellow stator wires, to each other and to Frame ground.
I don't know what type of voltmeter you have (there are hundreds of thousands of them and almost as many designs).
But, with the key switch on as well as lighting, measure the Black wire at the Vreg to see if it is different from the actual battery voltage; S/B within .5 V.
Do the same measurement at the Rectifier plug across the Red/Wht and Green.
You could also check the six rectifier diodes to verify they conduct in one direction only. The right meter and scale selection is quite important here, so refer to your meter's owner's manual.
Cheers,