The way I was taught to test generators per VWoA training center
c.1972-disconnect wires from the generator. Ground the field terminal. Put
positive lead of voltmeter on the positive terminal of the generator(larger
wire). Ground the negative of the voltmeter. Run the car. If the generator
is working, you will get as much as 30v out of the generator. Grounding the
field causes the generator to put out all it's got. Don't run it for long
this way,though. If the generator passed this test, we replaced the
regulator.Added note:some guys polarized the generators, others did
not(replacement rebuilds from VWoA). It didn't seem to matter. I guess it
depends on the magnetism of the filed coils. I always did it when I made a
repair. To do this, you ground the field of the generator, and add 12v to
the + terminal(fan belt off,silly)and the generator turns as a motor. A few
seconds is all that is needed. Also make sure you ground strap from the
trans to the body is good. This can cause a good guy to go crazy working
half a day trying to fix a mystery problem(I've seen it happen).

Have fun,
yug