shortbus:
In general there are three pases that are combined in the stationary part of the alternator, so you get 3 phase rectified output.
The rotor is the magnetic field that's changed. Usually the slip rings runs on two brushes, I've seen one at 90 and both at 90 degrees. Generally, that rotor is isolated. Not sure you have to get the polarity right. I doubt it, because of the rectification.
One end of the rotor is tied to +12 and the other is chopped to ground, so I think it can be a simple comparator, but you have options of: On/OFF, PWM, or current controlled.
Not exactly sure what drives the idiot light. I do know that if the wires to the rotor are severed (I had a weld break), the idiot light stays off even though the alternator is not working.
Some alternators (GM) had a test point, but getting to it was tough. You grounded a tap sticking out of the internal regulator and it forced the rotor full on or max charging current.
My quick and dirty tests are to measure the voltage drop when the alternator is good from the + of the battery to the + of the alternator with lots of accessories on: lights heater and AC and record it.
Then I usually throw a scope on it to diagnose problems. Most of the time you could replace the regulator and or diodes, but now - who knows. You find spot welded diodes, a bad bearing or bad insulation or that broken wire on the rotor.