Dave, what would you look for when testing with a meter?
I`d be looking for a signal that goes positive 12 Volts when the stereo was switched on, and Zero volts when the stereo was switched off.
What's the difference between taking the power from Power Antenna / Remote turn-on / whatever vs normal switched live 12v
Power antenna signal sometimes only works when the antenna is required, ie: radio switched on, as opposed to a REMOTE signal that is on when the stereo itself is switched on. If you are listiening to the CD, then it is safe to assume that teh REMOTE would be ON but that the Power Antenna feed may not be required (common on many stereos)
Either signal has a short time delay on it so that the stereo is switched on at a different time to the amplifier thus avoiding damaging voltage spikes and signal "thumps".
Aren't all the sources 12v?
Yes, but its the timing that is also critical. If you used an ignition feed, this would be turned on at the same time and would definately cause audio damage.
Is it sending a signal to the amp before powering it or powering it and then sending the signal......?
The correct wire is there, and if you use it, and wire it up correctly, then this question is redundant.