Alternator or Voltage Regulator?

[bigbrownbenz]

I've got an '84 190E 2.3 in the garage with charging issues. At first the only hint for a few days was the battery light coming on. I had just fiddled with the instrument cluster replacing some bulbs, I figured it was just a tempermental gauges since I didn't have any other electrical problems. But after a few days the lights started dimming and the radio switched on and off intermittently. I was pretty sure I knew a bad alternator when I saw one.

Sure enough, the battery tested good, but the alternator read as undercharging. So I plunked down the $190 at O'Reilly's for a new alternator.

But from reading some of these threads, it seems that the voltage regulator is more likely the problem. Is there a way to test the voltage regulator to see if it's the problem or if it is in fact the alternator itself?

Thanks,
Gabe