How do you all ensure correct polarity with aftermarket cables?

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.

Regards,
-- Al

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. 

I feel like I’ve stumbled on a piece of hidden information. I’ve been playing with stereo gear since I was a child and always thought of the + & - terminals as absolute concepts that needed to be followed strictly for proper use. You can imagine my surprise at hearing that there’s no industry standard for polarity?!?!?! 

Every piece of equipment I’ve ever used has these clearly demarcated terminals on both ends, with separate colors to make sure you get it right, how could it be that it doesn’t matter! 

My cheese has been moved MC, idk what to believe anymore. 

I have Sennheiser 650 with Cardas aftermarket cables attached.  On the connection to the earphones,there is a Cardas logo.....I make sure the logo is facing out.

Every piece of equipment I’ve ever used has these clearly demarcated terminals on both ends, with separate colors to make sure you get it right, how could it be that it doesn’t matter!

Terminals are clearly marked for polarity for the sole purpose to insure that both the Left and Right speakers are wired with the same polarity.