I really wasn’t expecting this kind of answer from you Chuck.
Surely there’s a definitive way of testing this?
Of course there is. Its just that it doesn't matter because as testpilot said there's no industry standard- and even if there was it only matters if you can hear a difference.
But anyway if you really want to know is which way is which you hook it up to a battery. Any battery. You can use a little 1.5V if you're worried or 9V if you're not. Either way you can see right on the battery which is + and which is - and then all you have to do is touch the wires and the driver will move one way or the other. It will help I guess if you can see the driver, maybe not so easy on headphones. Or on the other hand again like I said in the beginning you can just listen. Never done this with headphones, but I bet you will feel the pressure when + really is +.
Main advantage of this method is it shows absolute polarity all the way to the driver.