How do you know if a cable is neutral?

First, you can say it too. I would...

But obviously, some sales guys would say, that is the one with the most snake oil. It is also the one most expensive one. For some reason, I could guess that already. 

I went looking in another path:
I went between engineering and physics, found out that there is a mathematical relation between a cable's resistance and the Amp's DF.
As the Amp. is a given, all left is the length you need and than I can calculate the gauge.
Quite simple. I have an excel file for that. 
It would cost you way less than the snake oil, and sound much better.
Try it.
For those who have some box like on the cable they sale, it is not natural.
There is some sought of passive electronics that changes the sound.
Try to stay away from that.
Especially if along the box, they also got some snake oil with it, to make it even more expensive. 

Nothing is neutral.

Or Zanzibar....

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.....from Switzerland!

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.

It's from Sweden.

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.

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

Guess what... It's all relative!

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.

Yeah, what Bigtee said. Very cogent and accurate response to a great question! At the end of the day, there's only one (or two if your partner is also into it) opinion that matters: yours. Neutral becomes the reality you seek.

Bigtee has given a very good answer. I discovered that what I had thought was neutral wasn't really, when I heard a _really_ neutral system.

The gentleman who had put that one together had immensely more experience than I did, as a hobbyist, reviewer, retailer and distributor on two sides of the globe. To him, the term "neutral" meant the sound of specific equipment in specific combinations, as did "warm", "cool", "analytical", "musical" and more. From his collection, he could pull out pieces that demonstrated each quality. I was just floored.

He educated me so that I could tell him what I liked, and once I knew, it didn't matter to him what my preference was. One sound wasn't "better" than another, just different in quality. The notion that "it's a matter of personal taste" didn't just serve to make distinctions meaningless. After those sessions, I could name my own taste and know what it meant, and I never again thought it was better than anyone else's. That's what an education does for you, I guess. I've never stopped being grateful.

Thanks for the chance to share.

I think Bigtree's answer covers every peice of audio equipment. I crack up every time a reveiwer uses that term.

Million dollar Question

Hell of a good question, and hell of a good answer, Bigtee!

You really don't know. Each system the cable is inserted in will interact differently with a different possible result.
I have often wondered how you actually tell also. You would have to have a known absolute to compare it to which I don't think exists. One mans neutral is another mans colored.
Most of the time, the reviewer is comparing it to something HE has perceived as neutral in comparison to what he has gained through experience compared to HIS reference. Now, wether or not it really is neutral---can't say. Depends on HIS reference and tastes. Look how many reviewers use the same cable---hardly any! watch how many responses you get to a thread on cables with each person's "Neutral" recommendation.
I can say that a cable can only degrade the signal, it can't make it better. The cable that degrades less of the signal would be more neutral. Most people just go with the cable that seems to offer more of the attributes THEY like and label it neutral! Hell of a good question!