After starting

When the engine first starts, alternator B provides power to the engine, and alternator A is being verified but not connected to any load.  Once you reach about 2500 RPM (for a few moments) the Rotax fusebox switches alternator A over to the engine, and alternator B is assigned to the airframe. 

What you see when this happens depends on whether or not your start power switch is still closed. If you have a manual switch to activate start power, when that switch is closed the engine electrical system is tied to the airframe system. With it open, the engine and airframe electrical systems are fully isolated from each other. Of course in normal flight operation the switch is open and the engine electrical system is protected by isolation.

With this in mind, you can see that if you leave the start power switch closed after starting, alternator B will power the engine, but will also be connected to the airframe via the start power circuit. In this case, your battery should show some charge on the amp meter immediately after starting. Once 2500 RPM is reached and alternators switch, you may see a slightly higher charge rate, or you may not.  So essentially, with the start power switch closed, you may not notice anything when the alternators switch.

If you are in the habit of turning your start power off immediately after the engine fires (or your aircraft does this automatically) then what you see will be different.  Assuming the start power is turned off before you reach 2500 RPM, then during the time before the switchover there is no alternator power being provided to the airframe, and you may see a discharge (negative number) on your airframe amp meter.  Once 2500 RPM is reached, alternator B is assigned to the airframe and you see the charge rate go from negative to positive. I make a point to operate this way. I turn my start power switch off quickly after starting and before advancing to 2500 RPM. This shows me two things. First it proves alternator B can operate the engine without the battery being tied in, and second, I get to see the alternators make the switch by watching the amp meter. 

If you choose not to install a current shunt and ammeter, you can discern the same information by watching the bus voltage.  In Jeff's last paragraph, a discharge will be shown by a bus voltage at or slightly below the resting voltage of the battery (i.e. immediately after turning on the battery master switch).  When the Fuse Box switches the A system to the engine and the B system to the airframe, battery voltage will immediately rise to something higher.  The exact voltage you'll see depends upon system loads, engine RPM and battery state-of-charge, but normally something around 14V.

Like the avionics master switch, the ammeter is an installation that persists in aviation despite having long ago outlived its usefulness.  An ammeter tells the pilot virtually nothing of value that isn't communicated by the voltmeter, supplemented by active notification of low voltage (via an EFIS alarm or discrete circuitry).  Both the avionics master switch and the ammeter shunt (and all of the associated crimped terminals and securing hardware) are unnecessary wiring complications and single points of failure for everything downstream from them.

Not trying to start an argument; just my opinion.  Potentially worth exactly what you paid for it.

Thanks Eric, you are correct.  I used the amp meter as an example because that’s what many folks are used to, and my post was already long enough to be boring without going into both examples.  I would only make one argument for having an amp meter, it shows the rate of charge (indicating the load on the system). That is harder to visualize with a volt meter.  

"it shows the rate of charge (indicating the load on the system). That is harder to visualize with a volt meter"

I was going to say the same thing. With only V, you can find yourself asking "Wasn't that 13.9v a bit ago, or was it always 13.6? Guess I'll watch it for another 10 minutes and see what it does."

With an amp display, you can immediately tell if there's an issue.

FWIW, After starting my 912UL cold, ambient temp 84F, she idles at 1800 rpm, and the voltmeter indicates the same as before start, 13.1-13.3 VDC.  Four minutes after start, idle rpm has increased to 1900, and the voltmeter indicates 13.4-13.6 volts.  Increasing rpm to 2,500 for taxi, the voltmeter increase from idle is negligible, 13.6-13.7 volts.