How do you test the alternator on a 1973 VW Beetle?

Start the car and rev it to at least 1500RPMs. Use a voltmeter to test the voltage across the two battery terminals (under the back seat). If it tests out at around 13.8-14.5 volts the alternator is working.

If it tests out at 12.6 volts or below the alternator isn't charging.

If it tests out above 14.5 volts the alternator is charging, but the voltage regulator isn't working correctly.