From "The Best of Geo" this tidbit:
Some basic charging system tests (note that if your car is positive
ground, all the voltages are negative) :
Engine off :
1) Double check the mechanical stuff : fan belt present and snug,
generator snug on mounts, wires firmly connected to generator, etc. You
should only be able to turn the generator fan with great difficulty, if
at all. If the generator has been off the car, it's worth repolarizing
it.
2) remove the wire from the 'F' terminal on the control box
(regulator). Measure the resistance between the F and D terminals, it
should be under 1 ohm. If not, the control box is bad.
3) Measure the resistance between the wire that came off the F terminal
and ground. It should be between 5 and 10 ohms. If not, either the
generator or (more likely) the wiring is bad.
Put the wires back, Start the engine, and set it for a fast idle
(1500-2000 rpm)
4) Terminal E to battery ground post should be under 1.0 volts. If not,
you have a bad ground. Also check from E to the generator housing.
5) Measure D to E. This should be around 13-15v, if it is then the
generator is working. Higher indicates a bad regulator.
6) Assuming D-E is under 13v, compare it with A-E. If they are the
same, the ignition lamp should be out. If not, the wiring to the lamp
is wrong.
7) Also assuming D-E is under 13v, check D-F. It should be under 1.0v,
if not, the regulator is bad or badly misadjusted. One exception to
this is if the ammeter is reading a lot of charge at this point, 15 amps
or more, which could indicate a bad or deeply discharged battery.
8) As a final check, (again assuming step 5 was under 13v) stop the
engine, remove both wires from the generator, jumper the terminals
together and connect the voltmeter from either terminal to ground.
Start the engine. Rev no higher than about 750 rpm, and check the
voltmeter reading. If you don't get at least 15v, the generator is
definitely bad. According to the book, around 4-8v indicates a bad
armature or brushes, around 1v indicates a bad field. If you do get
15v, reconnect the wires and repeat the test with the F and D wires at
the regulator. If you get different results here, then the wires are
bad.