Trained ears.

What is complex is to tune the system like an instrument. You have a few components working together that must become the one. This does require experience and some knowledge in addition to hearing. And don't forget the room - it is probably the most important single component, provided the rest are good enough. One wrong set of cones under one component and the system is out of tune to various degree, wrong platform under turntable and it's no good, wrong or not quite right power cord on your Studer deck and something is lacking.

First make clear you have a balanced system. All music should sound eaually fine (or bad). The system should not favourize any specifical type of music. Make sure every recording you play sounds different. That you can hear differentiate between good and bad recordings. That you can separate individual instruments and singers and follow their lines. And that the transparence and dynamics makes you get a "live" feeling.
Then we have the question if it sounds correct. Play the piano someone says. I probably have a piano on thousands of recordings. No one sounds the same. What is correct? If it sounds natural it's ok. Only if you were there and did the recording yourself you would have a chance to know. 
Read between the lines. Lots of exaggerations when audiophiles discuss "huge" differences between dacs, amps, IC's whatever. In reality very small differences, maybe probable to hear if you really want to and are searching for them. Fun, but not sure if it really matters.  

Agree to all , N80 your number 4 is very very very true Listen listen listen listen listen, spend time listening, If you have two systems helps too....

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.