pinstripe...,
The best way to test it is to eliminate it. You didn't say what year or model you bike is as there are two setups. One is '71 to '79 and the other is '80 to '83.
If an early model unplug the regulator plug of the reg /rect, run a jumper from the battery + terminal to the green wire brush.
If a late model, then run a jumper from ground to the green wire brush.
In both cases, after you have run the jumper, start the engine and check the battery voltage. If the battery voltage climbs steadily to 15 volts, then the regulator is bad. If the battery does not climb steadily to 15 volts, then the regulator is probably OK and you can look elsewhere for the problem, starting with the rotor resistance.
DO NOT ALLOW THE BATTERY VOLTAGE TO CLIMB ABOVE 15 VOLTS!
With the regulator bypassed, the voltage is...well...UNREGULATED and can climb to a very high and destructive number. The jumper is a test only...you cannot run the bike that way.
To test the rectifier part of the reg/rect, use an ohmmeter and measure between each white wire and the red wire, then reverse the test leads and measure again. One way should read a low resistance, the other should read a high resistance. The high should be at least 10 times the low resistance.
Then, do the same tests between each white wire and the black wire.