I think we know when we are playing something we are very good at playing or playing something which we have difficulties to keep under our fingers. Any piece which has a small physical or musical difficulties have to be ironed out, practiced until they become automatic otherwise they become pieces which are "difficult" for us to play.
Improving the quality of your pieces you play on the piano is constant. The more you play the piano the more obvious these changes are. People who are dedicated to their piano study can always say, last month I played this peice much worse than what I did now. This improvement never ever ends. There are hundereds of instances in any piece where we question ineffective physical touch or sound production. Some people cannot pin point these things so they say that perfection is unattainable and impossible and non-existant. I feel that yes, an ultimate perfection, where nothing can be proven to be "better" than it is impossible. Unless God plays the piano for you. But I like to think that we all mirror God, especially the love/creative side of humans. Once you learn to pick out things which cause you problems physically or sound production wise you move closer towards perfection. I personally think I can play a few peices perfectly simply because I have played them all my life and know them inside and out, know many different ways to express it, know how to balance expression to something that is ideal in my mind.
I think you have to let the music play you for a little while, then you turn around and play it! This means, let the music overwhelm you, get extremely passionate with it, over do all expression, play with huge contrasts over exxagerations. Then after all this steam is released go back and look at it with a critical musical mind, listen to lots of advice from people more experienced than yourself.
The problems I find in music is that being a "perfectionist" can be dangerous. You can get so caught up over trying to make something perfect that you flounder about and learn slowly. Music is art, art works best went we are in a positive frame of mind, if we are in a negative mindset our music work will amplify that emotional state and even reflect it in our playing. Getting upset that our playing is not perfect or how we would ideally like it wil not help us, but interlectualising it does. What part of your playing don't you like? Why? What do we have to do to change it? How will that change push the music closer to perfection?
I always ask these things when I practice even if they are not a conscious effort anymore. I find it exciting trying to discover the best sound and physical action, I really think that frame of mind pushes you to perfection much faster than getting upset that you cannot play the way you want. Search for perfection with interest, don't say it is impossible and unattainable and use that as an excuse for second rate playing. I like to believe everyone can play at a concert standard. I really think anyone can if they work hard and with the right instruction.
Perfection simply is entertaining playing in my mind. I think people play close to perfect when they play and nothing provokes listeners to think or say, "Hmmm thats not exactly what I would do." If we hear someone play and think, "That is not what I would do but it sounds great." This is a peformance which changes how we observe music, and this is something only "great" pianists can do and which is to me perfect. Can you do that for everyone? Of course not. Some people sit up to attention when Horowitz is mentioned, some people wave his name off with a grimace. Most important is that you please yourself that is why we study music afterall! But some of us are our own toughest critics i guess that is what makes a good pianist over a mediocre one. Someone who is never satisfied with how they play and make changes to their music until they find something closer to their liking. Still I have always heard this advice echoed from many great teachers of art/music. "A good artist knows when they should stop."