On the "A" bikes, the regulator sets the field coil current, which then excites the stator coils to generate power. So, a PROPER test would include bypassing the regulator directly to the field coil, then checking the stator output. If the field coil is shorted (rare), he would have been confronted with melting wires: if open (less rare), he would have noticed (if he bothered to check) that no current was flowing to the field coil. And, when no current flows, no stator output is possible. That's the proper test method for these bikes.
Although I hate to even say it, today's bike shop wrenches blow, if it is electrical. They will say anything they can to get it off their bike lift because they are often paid on commission, part of the labor, and have absolutely no clue how to fix or test electrical. His comment of "not available" should have been the first comment before charging you a dime for an exam of it.
If you have friend available nearby who can read a schematic, then do this test:
Disconnect the regulator.
Jumper the field coil's wire to the bike's BLACK wire with a temporary jumper.
Put the meter leads on the battery (on DC volts range) and start the bike. Watch the battery voltage for a rising reading when the bike is held at 2500 RPM for about 1 minute or so: it should climb upward in volts if the alternator is working.
Shut off the bike, then connect the meter in place of the jumper wire above, as the meter to AMPS range (more than 2 amps) and move the meter's test leads (usually just the Red one) to the "A" terminal so the meter will read amps. Turn the bike's key ON, see if you get a reading. It will be an amp or two more if the field coil is not shorted, but more than 4 amps if it is shorted.
Troubleshooting techniques:
If the bike's main fuse (15 amps) blows when you turn the key on with this jumper in place, this indicates the field coil is shorted (i.e., if you have jumpered it correctly). If the fuse blows after the bike has been running for a minute or two at 2000+ RPM, the field coil may be partially shorted, and the battery voltage will fall downward during this test.