The regulator/rectifier has four diodes in it arranged to function as a Full-wave rectifier. Since it is a sealed and potted assembly there is no way to check each component inside individually. If one of those four diodes are open or shorted the entire reg/rectifier is junk. So in order to diagnose one of them you must first verify that you have high AC voltages (AC voltage and current increases as RPMs increase) being supplied by each of the three alternator windings (its a three-phase alternator) in the stator. At idle speed, the AC voltage going into the regulator rectifier should read a minimum of 17 volts AC for this system to function, due to voltage drops (1.5-2.5 volts depending on the type/quality of each) inherent to the function of those diodes, plus the additional voltage drop that can be expected through the regulator section following those diodes.
I don't know what the factory AC output spec is claimed to be at idle speed on any Honda... only that it must be at least 3-4 volts higher than the minimum DC voltage expected on the output for the reg/rect to be functional, due to the losses all of those internal components are known to exhibit as current passes through them.
So first step in diagnosis is to tap into the AC supply and measure it where the stator plug goes into the reg/rect unit. If insufficient AC voltage is measured there then check each stator winding with an ohm meter per the manual. If sufficient AC voltage is being produced there, then measure both AC and DC voltages at the output across the battery terminals. If AC voltage at the output is much above 15-20 millivolts at idle speed then the reg/rect is probably junk, no matter what the DC voltage measures. If DC voltage at the output is below 12.5-13 volts DC at idle speed with a good, fully charged battery installed then its probably junk. DC volts readings above 12.5 volts (up to 14.5 volts DC) at idle speed with a good fully charged battery installed and with no lights or accessories turned on is the normal expected operation of the system. Max charging voltages normally should not exceed 15 volts DC at any time throughout the operating RPM range of the motor.
If voltage at the output ever exceeds 16.5-17+ VDC the CDI and/or the ECM can become permanently damaged, so it is important that a battery in good condition is installed and all cables, terminals and connectors are clean and tight at all times.
Have fun with it and welcome to the forums!