Trained ears.

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. 

Just like athletes who have to trainrf their bodies, critical listeners need to train their hearing.  Wide exposure, frames of reference, concentration, attention to detail...repeat, repeat and repeat.  No different than how a wine sommelier or perfumer train their senses. It takes both an interest and a commitment.  

If you know the sound of live, acoustic instruments, it's not that difficult to evaluate audio components ... especially if you have a few recordings that you've made yourself.

If you are comparing equipment, use familiar music, and do an ABA test.  When you go back to the A after listening to the B, you'll really hear any differences.

gawdbless

Good advice as above. A piano is both melodic and percussive in nature.
Test these characteristics with familar music, passages, for accurate texture and timbre when evaluating electronics. Ask yourself, "does this music reflect the live event"?  Possessing the ability to tune a piano requires an awesome ear. You already have an awesome ear. And yes, you can't tuna fish.  Enjoy the music and have fun.

Happy Listening!