How does one know that Gen A has clicked on?

Thanks, Jeff,

Just to make it perfectly clear for everyone...

If you see >13v on the Battery/Bus with the START POWER and the BACKUP POWER Switches in the OFF position,
the A Gen is online and powering the ECU and the B Gen is Charging the Battery.
If you see <12.6v on the Battery/Bus, The A Gen is Offline and the ECU has commandeered the B Gen to keep the Engine running at the Expense of the House loads.
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Secret Backdoor Emergency Workaround...

The B gen has more than twice the capacity of the A Gen.
In the event of a Gen Failure, it behooves you to get on the ground soon.
It is NOT an emergency but it is at least an urgent situation. Flight to the planned destination is still possible.
It is assumed that your battery will power a limited set of House loads for the time it takes to land.
However, by activating the BACKUP POWER Switch BEFORE the battery has had a chance to discharge appreciably, It is possible to "Borrow" some of the excess B Gen capacity that the ECU is not using and "Maintain" (Not Charge) the Battery and House loads indefinitely.
Turn OFF any unnecessary equipment so that the Bus remains above 13 volts (13.6 is an ideal target)
NAV and Collision lights, Music, Passenger Intercoms, Heat. AC, Transponders, ADSB, etc, can be powered down until needed when arriving within the destination airport environment.

This trick needs to be performed soon after discovering the problem.
If you discovered the problem because the LOW VOLTAGE Alarm announced the Battery was below 12 volts; That would NOT be the time to be stealing Gen capacity to try to recharge it.