Trained ears.

When i began my journey on a another, enjoyable, frustrating, subjective hobby - wine -- i asked a great sommelier the same question. His answer: pop a lot of corks, and think abotu what you are tasting".
I learned not onyl to taste wine and what was good, btu more importantly what i liked and what it might do well with.
G

@gosta : You bring up a good point in regard to transparency. For me that was one of the first things I was really able to articulate about what made me like hi-fi SQ. With my system the speakers disappear. It does not sound like the music is coming from a component or machine, it is right there in the middle of the room.If you close your eyes you forget about equipment and to me that is right up there at the top of my list of important things. And I still forgot to mention it above........

This one is also easy to test and does not really require 'golden ears'. Put on a well recorded song, medium volume. Stand up. Cover your eyes. Spin around a few times until you are disoriented and then try to locate the source of the sound. It should be somewhere between and out front of your speakers.

@tomic601 :  "Anybody posting on this board is off the deep end...."

Agreed. And when someone sees you spinning around with your eyes covered pointing at empty space this is confirmed.

The n80 spin test as it shall now be known !!!!
freaking love it...

going to try it sober
first....

@tomic601  :  "going to try it sober
first...."

Safer. But generally not as fun or revealing.