How do you know if a cable is neutral?

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!

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

Million dollar Question

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

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.