Experience.
Actually it is very hard to be able to state with 100% certainty anything about 'a' specific item. Just because nothing is heard in total isolation. it is always in conjunction with other items.
So what one needs is a bunch of stuff one KNOWS well.
Starting with recordings, Finding recordings which give a wide range of specific types of things, treble, female vocals, male vocals, Saxophone, piano, bass, mids, massed violins... all the things you care about, and then some you do not, but know can be important. Then they also have to 'sing' to YOU, that is you NEVER tire of hearing them.
and continuing through various electronics one has heard in other combinations. So you have an idea of what each of the electronic parts sounds like in a variety of situations.
I have two cable sorts i have heard through a lot of variations: a handmade cable with "Mark Levinson" wire, and Kimber KCAG. (And a third I currently do not use, Kimber PBJ)
So these cables i can say I 'know' what they do. The Levinson is very much like original Cardas, warm, soft focus, The KCAG is clear, a little bright, almost. The PBJ is the most neutral, but a little fuzzy. My replacement for the PBJ, Kimber Hero, is the same but not fuzzy, only I have NOT used it on several systems to say I 'know' it.
So here I have some wire I know. and now I used my current system for a year or so, along with the digital stuff i have had for many years...
So I can decide where to use what wires.
All my digital uses the Levinson wire. And the phono the KCAG, the outputs are Hero.
Is my system neutral? NO. it is a little thin, due to using so many power conditioners, I trade off the clarity I want, for a little thinness in the lower mids.