There really isn't any way to test the Rectifier/regulator by the wires. The regulator portion inside of it redirects power back to the stator to keep from overcharging the battery once it hits around 14.5-15 volts DC. You will get numbers on the multi-meter, but they don't really tell you anything.
The only real test for it is done by testing the VAC output of the stator, which should be around 40-45VAC at 2000-2500 RPM, and testing the voltage at the battery. It should be around 13-14VDC. If stator tests good and battery voltage dwindles down to 11.5VDC or lower, then the rectifier/regulator (VR) is bad.