sorry .oops! output is checked by inserting an ammeter between the output of the dynamo and the battery, normally it is practical just to lift the neg off the battery and connect the ammeter to that lead and the battery.
If it is just the voltage you want to check just put a meter across the battery , if the unit is ok you should see 7-7.5volts if it is ok
Many users of older bikes with dynamo’s replace the old mechanical regulator for a fully electronic device fitted inside the original case , it’s a bit more efficient. Wassell’s make a nice little unit, 6 volt positive earth