Whats the best way to test the stator?
Measure the resistance between the two yellow wires and that they are not shorted to ground...........Should be essentially 0'ish (zero) ohms with standard DMM between the two yellows' and open (high resistance) from the yellows to the motor block.
If all is good above and between the yellows you see 0'ish ohms, the winding's themselves could still be shorted within their coils........To check that (other then visual of char and smell), fire up the engine and measure the AC voltage this time between the two yellows again. At an idle and unloaded'ish, ya might see around 12VAC, rev it up for a short bit and you should see perhaps around 20VAC.
If you're getting reasonable AC readings but, still no DC out higher then the batt when rev'ing a bit, that implies the reg/rec. and/or it's wiring. You say, you have the reg's small red wire capped off/dis-connected? That might be the issue (not sure, some designs require the sense wire be connected, others don't). Tie that off as Faz said, "to the purple" (not sure where that is) or tie it where the reg's big red wire ties, right to the battery+, perhaps electrically, they're one in the same point on your motor. If so, moot point.