How can you test the alternator to see if its good

ON or OFF vehicle? Special test equipment needed either way , but if it's easy to take off , have it 'bench tested' Without an ammeter on car testing (especially old-school way of disconnecting battery) would be unreliable. You can have an alternator charging at near correct voltage under no-load conditions , but not able to produce the needed current at higher load levels , so voltmeter alone , is also unreliable.