stick a screwdriver or something to the rear bearing on the back of the alternator. If its putting out it will be magnetized and stick.
What kind of alternator is it? Delco 10SI with internal regulator maybe? If so it would have either been built with a 6 volt self exciting regulator that excites when your alternator rpms increase or a 6 volt non-self exciting regulator and you will need an exciter wire to the alternator to excite the regulator.
