Normal charging voltage for most cars is 13.8–14.4 V, but measuring voltage alone does not prove an alternator is healthy; full testing needs a bench.
For most cars an alternator output of 13.8–14.4 V is normal regardless of engine speed and electrical load; exact figures are in the owner’s manual.
Measuring voltage alone is not enough. An abnormal reading usually points to the regulator, while other faults may leave the voltage normal. Modern digital regulators can also fail partially: one that stops sending its feedback signal to the engine ECU keeps the alternator in constant charging mode. This shows up neither on the dashboard nor in a scan tool, but on long trips it overcharges the battery and shortens its life.
MSG Equipment recommends test benches that check all output characteristics and offers models ranging from small workshops to large remanufacturers.