How do you all ensure correct polarity with aftermarket cables?

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.

testpilot

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.

That is mistaken. There can be other reasons that it's necessary to maintain polarity.

For example, there are servo-controlled woofer systems that require  the amplifier output to have the same polarity as the preamplifier output. Major problems can follow if the two are not in agreement.