There are some very concrete things to listen for in determining neutrality.
- Resolution is the most obvious. Without resolution we can't achieve a 3-D immersive soundstage, can't get to a "you are there" presentation, timbre between different instruments is less distinct and real sounding, and vocals sound dull. This is especially important for interconnect cables, because one ic that smears detail can bottleneck an entire system. Very few copper cables are capable of enough resolution, and many that do have other issues. Jorma is probably the best copper cable I've ever tested.
- Lack of fatiguing artifacts is another. Over time it's possible to recognize and categorize types of noise, distortions and artifacts. Some examples are grain from stranded wire, accentuated leading edges from impure silver, there are many more. It's very important to reduce fatigue to a point your stereo doesn't annoy you over time. Also, these things tend to reduce resolution and clarity.
- Tone is a tough one because it's very system specific and there's a wider range of personal preferences combined with a much more subjective aspect to tone vs resolution and fatiguing sounds. This is why I offer choices of IC and PC that differ in tone without one necessarily being "higher end" than another. The thing to remember about tone is warmth smooths over detail and adds a sameness to the sound. OTOH, it's important to have enough warmth to provide realism.
So tone is hard to categorize as neutral, my D4 has been characterized as both too warm and too cool about the same amount so I think it's pretty neutral overall. D5 otoh is consistently characterized as warm. So these things are subjective to a degree for sure, but there's also certainly a lot of statistical agreement on the character of a particular cable.
In doing a lot of listening tests I'd also say that different people are sensitive to different aspects of audio playback, but the one common theme is preference for a 3-D immersive listening experience and that requires a fairly resolving system. There is a psychoacoustic barrier to achieving this that has to do with the system being able to reproduce the detail, the room not butchering it, and your brain as perceiving the detail in the recording as dominant vs the sound of room reflections. This is why Harman has determined that off-axis sound is so important, when reflected and returned to your ear it is closely enough aligned with the direct sound so as not to distract you, and this allows your brain to piece together the soundstage from spatial cues in the recording vs reflected sound dominating, and it also allows timbre to remain more distinct vs speakers that have poor off-axis response.
So, my goals are to offer a cable with enough resolution to achieve a 3-D immersive presentation, closely followed by not creating fatigue, and then having a realistic tone. In over a decade of sending cables to folks to try out it is tone that is the wild card, but I'd also say that over time and as a person's system improves they almost always prefer less warmth. Over the last decade or so my D4 has been around the quality of gear has improved enough I've slightly changed the D4 to be a bit less warm by reducing gold content a fraction of a percent. This is the ONLY way this cable is "voiced", otherwise it's design is purely objective. Some of my other cables (the silver ribbon SC and PC) are not "voiced" at all, they are simply the result of designing the most objectively perfect cable possible, and unsurprisingly, this results in a furthering of the goals I've stated previously.