Does anybody know how to test the voltage regulator? I figure I should test it prior to buying a new alternator.

Swap with a known good one. You can get a solid state replacement that is claimed more reliable. AFAIK, there is no test, or at least no simple 5 minute test. I'm sure you could pull out the multimeter and oscilloscope and test all of the component.

Mine failed visual inspection. Take it off, turn it over, and take a look. This test required no tools other than eyes and brian that recognizes RUST. Yup, it was super duper rusty underneath. New one = no more drained battery.