If your voltage regulator is working, immediately after starting or on low battery condition the alternator will be charging at 13.5 to 15.2 volts. You can check the voltage at the battery lug of the alternator, and again at the battery, to see if you have voltage drop. You need more than 13.5 volts to charge the battery.

After the battery is fully charged, the alternator will stop charging (stock design). Then the battery voltage will be lower, and should be no lower than 13.5 volts. That's nominal voltage, anything lower and you may be stranded. 12 volts is a dead battery.