Trained ears.

This is a nice tool to play with.

http://harmanhowtolisten.blogspot.com/

Complicated. First of all I try to figure up what annoys me most in what I hear and then go from there. Drive flow and coherence are what should be done right before everything else, I believe. Assuming that speakers are more or less okay, this means that you start at the source. And the best source is tape.

Agree with the above posts, plus sitting in the sweet spot with eyes closed helps quite a lot. Also, if you wear glasses, remove them.

Tom

I agree with all above posts.  Listen to music that you are familiar with (or that you like and are going to listen to over and over).  Do the instruments sound real?  I’ll listen for drums...high hats / cymbals.  

Familiar music is key, critically listen, close your eyes.  

If everything is right, or close, you can go from critically listening / evaluating to just getting lost in the music.

If you’re trying to see what differences a wall outlet or power cord makes make sure to change only 1 thing at a time.  Power cords and outlets / power conditioners increase bass depth and weight, gives a darker background from which music seems to just appear and gives better highs in my experience.

It’s all system and synergy dependant but changing out pieces and then evaluating the difference is part of the fun.

I wonder if people really learn to listen.  I was mesmerized the first time I heard my uncle's JBL Paragon/Mac system back in the 60's.  My father owned EV400 and an HK receiver and I was constantly listening until I was old enough to buy my own stuff.  I think you either relate to hifi or you don't.  I guess you can learn to hear what system comes closer to a live piano, assuming the recording is remotely trying to accurately reproduce the piano.  But ultimately, I think it's a more visceral, subconscious thing that listening to a great system does.