here i'll break it down for you, i'm going to assume you've already tested voltage at the battery and found that it doesn't climb to charging voltage (~13.8-14v when revving over idle)
unplug the stator connector, then start the bike. with your multimeter set on AC volts (the V with the ~ on top), check volts between each of the yellow wires (there are 3 wires, so check 1-2, 2-3, and 3-1)
at idle you should see around 20ish volts, with the engine revving you should see around 50ish, maybe a bit more. all three of these voltage readings should be similar, if one is very low the charging output of your system will be much lower than it should be.
if the stator checks out good, it's the RR, if the stator checks out bad, change it before you change the RR. there are MANY MANY parts sold and replaced that weren't faulty, people just pour money over their problems.