How does one know that Gen A has clicked on?

Does it really matter ?

At some stage you are going to....

Taxi

Do a run up

Go flying

All these probably require more than 2500 rpm for 5 seconds

Surely that will be enough ?

Once generator “A” takes over engine functions, generator “B” is connected to the airframe electrical bus.  Most aircraft have a separate volt meter on the airframe bus, and when the generators switch over you will see the voltage come up from battery voltage to charging voltage (generally 13.x volts).  You should also see the amp meter (for the airframe electrical system) go from negative to positive (or at least to zero).    

I recently worked on an aircraft that had a failing alternator “B” stator.  The stator coil was going to ground, but it was still producing enough voltage to run the engine functions between the time start power was removed and the engine switched to alternator “A”.  However, once that switchover happened and alternator “B” was connected to the aircraft, the airframe volt meter never rose above battery voltage and the amp meter stayed negative (the battery was being discharged). Seeing that reminded me to pay attention to these indicators. 

Glenn Martin wrote:

Does it really matter ?

At some stage you are going to....

Taxi

Do a run up

Go flying

All these probably require more than 2500 rpm for 5 seconds

Surely that will be enough ?

a) It's a new engine, first start is today, want to verify it's working properly

b) Just because I do a runup for >5s and go flying doesn't mean Gen A has ever switched on

Before Start, Gen "B" and the ECU Internal bus are connected to BUS/Battery via START POWER.
You know this because you have seen that the Gen B LEDs along with all the other LEDs in the fuse box are illuminated and the A side is Not.
Immediately after START, Gen B is going to back feed out that START POWER connection (X3-2) and the battery will immediately show a charging voltage (>13v).
Once the "A" gen comes online,  the B Gen will transfer to the BUS POWER Connection (X3-3) and continue to supply the Airframe Bus as if nothing happened.  Nothing to notice.

Now comes the part where you Disable START POWER.
If the B Gen is still feeding the battery through START POWER, the Bus/Battery Voltage will immediately drop to Discharge levels (<12.6v).
If the A Gen has come online to power the ECU Bus internally and the B Gen has transferred to the BUS POWER Connection, the BUS/Battery Voltage will remain above 13v. Again, Nothing to notice. 

- - -

Jason,

Before the first start, Crank the engine for 5-10 seconds with the Lanes ON, Fuel Pumps OFF (No Start), and monitor the OIL Pressure to confirm you will have it (>20psi) when it actually does start.

Expect MORE than a little difficulty getting it to run the very first time.
Read THIS Thread first.

Bill,

Thank you for clarifying/correcting my post.  The explanation I gave assumes that the start power will be switched off immediately upon startup, before the alternators switch, which is not always the case.  Most of the time that's what I do, so I end up observing the alternator switch-over as indicated by the battery returning to charging voltage.