Get a decent digital multimeter that also measures amps, then place the probes across the battery posts.
With the engine running at say 4000+ RPM, you should be getting at least 13V or more, but not more than 15.5V.
If you're not getting 13V, you may be having either a stator problem or rectifier/regulator problem.
The stator is the part of the alternator where current is produced.
The RC51 uses a 3-phase alternator which is rectified to DC, then regulated to required voltage range.
The regulator/rectifier is the finned module on the left subframe area.
The stator cable connects to it.
By referring to the service manual, you use the resistance measurement portion of the multimeter to measure resistance of each stator coil, plus each coil to ground.
Each coil should be around .2-.5 ohms (although mine measures .6 ohms with no problems).
If a coil is below or above the required spec, the coil may be bad.
If you measure a coil to ground and it does not show an open, then you have a shorted stator coil.
If a coil is out of spec or is shorted to ground, you'll have to replace the stator assembly.
If the stator checks out and your battery charging voltage is out of spec (too low or too high), then you may have a bad rectifier/regulator module.
If the charging voltage is in spec and the stator checks out, you can perform the leakage current test on the battery.
If the current leakage is greater than .1mA, you may have a short in the wiring somewhere.
It's all in the service manual. The PDF version is here if you don't already have a paper copy.