This is AC, there really is no ground.
Be sure your meter is set for AC voltage, and on a scale that will handle 150 volts, if your meter is not auto-ranging.
Use alligator clips to connect one of the yellow wires to either probe on your meter (This is AC, so polarity is irrelevant) and connect the other probe to one of the other yellow wires.
I highly recommend the use of alligator clips since a revved-up stator can crank out over 100 volts, or the equivalent of sticking your finger in the wall outlet! Safety, safety!
At idle, you should get around 20 VAC, and revved up, you should see over 50, more likely around 70. The exact number isn't too important.
Do this for all of the yellow wire combinations, three tests total.
Then shut the engine off and set your meter for ohms and put one of your meter probes to something on the bike that's well grounded. Touch the other probe to something else on the bike that's grounded, just to be sure your meter reads less than a few ohms.
Now touch one probe to each of the three yellow wires in turn. If you read anything besides infinite ohms, your stator has in internal grounding, and it's toast.
Currently Running Stable:
2010 Honda NT700V
1986 Shadow 700
1986 Honda Trail 110 (Postie Bike)
1987 Honda Rebel 450
1973 Honda Mini Trail 50