What would you use

Well, first of all, you have to know what the piano sounds like in the first place. Anyone who has ever heard a Steinway grand knows the unmistakable "Steinway Crash" in the midrange sound when heavy impact chords are being played. A nine foot Bosendorfer has a signature of unmistakable bass power with less noticeable hammer impact. No midrange crash. A pianist or piano aficionado can tell instantly whether a grand piano or an upright or spinet is being played, by the resonance qualities. I could tell you with electric keyboards whether it is a Roland, or a Yamaha, or a Kurzweil, or with synths, an Oberheim, or a Prophet, or an Arp, etc. It is simply a matter of experience. Obviously, the better the recording and playback system, the easier it is to tell what is being played. But if you don't know the sound of the instrument, then you'll never tell what it is by the recording. Most guitar players and even keyboard players like myself can tell when a Les Paul, or a Telecaster, or a Strat, or an ES 335 is being played. On acoustics, a Martin sounds different than a National. Etc,etc....

Twl above speaks for me, too. To add a technical note, it helps if the system correctly reproduces harmonics, timbres, has pitch... Of course, a better recording helps!

...that's what i've started to realize.
for ex: i could now distinguish Yamaha from any other keyboard; I could now distinguish ES125 from any other guitar and certainly upright piano from grand.

Electric instruments can be very tricky. It may sound like a Wurlitzer electric piano, but one would be hard pressed to say whethers it's a real Wurlitzer or is it a Korg Trinity (or any other number of synthesizers) running a Wurlitzer patch? Solid body electric guitars have very little sound of their own. The pick up design and the amplifiers used are a greater determinate of the final sound than the guitar itself. Couple this with the choice of microphones and placement and a Telecaster can easily be made to sound like a Les Paul (Led Zep I & II).