Mine was pretty intermittent. Quick check with a multimeter would have shown it as fine unless you lucked on a fail time. Without a standard voltmeter, wiring one temporarily so you can keep an eye on it while driving might be best. One other hint with the Monaro though was when my alternator is charging it makes a high pith whine, when not charging it was quiet. Can only hear it when stopped though. Think it must be normal, both my alternators make the noise and I've heard the same on youtube videos.
What you could do is to is get hold of a cigarette light plug and place a couple of wires in it, approx 2 meters, wire it to a twin connector block, and temporary connect a digital Multimeter,
and then plug in to your power outlet [assuming you have one in your Console]
As for the alternator whine, haven't heard them on VT's > VZ's
But have on the Early VE's,
that Monaro Whine doesn't sound right, sounds bit like a bearing
If you can get hold of a Mechanics Stethoscope, listen to the Bearings at both ends of the Alternator,
if they are noisy, you should able to hear the whine.
On the other hand, if you could get hold of a Tech2 or a Alternative unit, and look at the live Data at the ECU and/or BCM,
that will probably explain why the dash has a hissy fit.