How do you know if a cable is neutral?

Don't the neutral ones say "neutral" on the label or packaging? :)

i had an experience w/ pwr cords- the after-market cord was a little brighter sounding/harsher than the stock cord; BUT, it did reveal a little more information. i think if a cable/cord doesn't give the listener the impression that it "rings", especially in the upper mids/low treble, it's a good choice. if your speakers or acoustics emphasize these frequencies, your cable choices are going to narrow. but then, your main fix lies elsewhere anyway.

or.....from Switzerland!

Or it's from Canada, in the context of the current conflict.

Actually in my HO, Ensemble gear is pretty neutral, and it is in fact Swiss.

Or Zanzibar....