You unplug the yellow pair of wires that feed the RR, and then test for AC voltage at idle, and up to maybe 6000rpm. If you need to check it for loading, maybe connect 300W of incandescent lights to those 2 wires. the 2W stator on the older bikes were rated for 350W output (more than enough capacity to run the bike!). Voltage starts around 20-ish VAC at idle, and is proportional with rpm from there. Not sure how much at 6K, but probably enough to light the lights, and also receive a shock, so BE CAREFUL!
Go to the electrex or electrosport.com web page. There's some pretty decent trouble-shooting procedures there.