How do you know if a cable is neutral?

Cables all do something to the transfer of information. If what they do is not something our minds can convert into harmonically acceptable information, then it sounds inferior to a cord that only deals with information which can be acceptable in the real world. Aftermarket cords either smooth out the sound or make it richer. Generic cords add non-harmonic info that any critical listener finds to be 'wrong'....because it just doesn't fit.
The harmonic series fo any note is the fundamental, octave, third etc.....if you add something outside of that pattern it wont fit.

Guess what... It's all relative!

I could have asked another way; Can we ever know if a cable is neutral?

Especially in context of cables, I have never experienced a neutral cable. Every cable I have ever heard has an identifiable frequency balance, along with its own speed, resolution, noise floor, articulation and a group of other variables. Probably the closest you can get to a definition of neutral is that the frequency balance seems evenhanded and that no frequency band sounds obviously exaggerated after extended listening. Oddly, many cables labelled as neutral have very little midrange but extended highs and lows. This is not the way music sounds and cannot possibly reflect the music on the disc. Nor does music sound dark or analytic or consistently cool. These are attributes of playback equipment and are therefore superimposed colorations.

It's from Sweden.