My current way of finding out what I might like or dislike is to play the P.Creek FantomX Ultimate Grand file, then play whatever else I want to listen to next...

So far, few of even the big GIGA sized VSTi's impress me as much. Listen to the detail on the inner voices during the quiet passages (anyone can bang away and sound OK), listen to how you can actually make out each note the file plays on denser chord voicings, listen to how warm and intimate it sounds... Now listen to your NEXT choice.

Are they still there? If so, you've picked a good one. If not...

Oh, and BTW... Every few weeks (sooner if you can), go and play a nice big Steinway, Yamaha, Kawai, whatever... remind yourself what a REAL good piano actually sounds like. You'd be amazed at how warm and intimate it sounds when you aren't hammering away at it. Unlike so many sampled sets.