How do you all ensure correct polarity with aftermarket cables?

Hi Everyone, 

I recently got some upgraded cabling for my Sennheiser HD6XX, and unlike the stock cables which are labeled L/R and make it obvious which way they’re supposed to be plugged in, the aftermarket cable has none of these conveniences. L/R is easy to determine of course, but phase/polarity is a little trickier. 
I’ve tried using this tool here:

The test for this issue is the center/twisted sound file, but I find the description and experience kind of subjective.. how am I supposed to know if it sounds centered, or twisted up in my head lol. This is a lot more straightforward with speakers, since you can just point them at each other and flip polarity until you hear significant canceling and then you know you’ve got it wrong and need to change one speaker. 
How do you all handle this?
Hey Robert, 

I’m able to push my ATH-AD700’s together like that, but the HD6XX headband isn’t being very cooperative. 
If I understand correctly what you are trying to accomplish is verifying that the new cable isn’t wired with a polarity reversal. Especially a reversal of one channel relative to the other, per Testpilot’s comment and per the stated intent of the "wiring" section of the writeup you linked to.

If so, and even if you are concerned about absolute as well as relative polarity, it seems to me that all you have to do is to disconnect the cables from the phones and use a multimeter to verify that the wiring of the new cable is the same as the wiring of the old cable.

And if by any chance it is physically possible to insert the cables into the phones with two different orientations, 180 degrees apart, and if the plugs don’t have any markings that would allow you to keep track of which is which, you could temporarily put a small piece of tape on one side of the plugs to keep track of that.

-- Al