Standard quick test for years was to pull a battery cable. If it died it was the alternator or regulator. Before internal regulators you could isolate which by running a 12v jumper to the field terminal but just for a moment because it threw the alternator into max output, if it was good, causing it to growl and usually the fan belt to squeal. I have been told one should never do this on computer controlled systems. On many occasions I took the battery out of my 68 shop truck with it still running and traded places with a run down one on a vehicle we were picking up. By the time we got back to the shop it's battery would be charged up if it was good.
