Reflecting once again on the question opening this thread ("How does one know...?), I realize that it is largely a question of self-confidence or self-trust (or of being true to oneself). If you enjoy self-confidence/self-trust, if you value being true to yourself above everything else, then you just know that your piece is worthy of performance (provided it succeeds in doing what you intended it to do). You don't need anyone else to tell you whether it is worthy of performance or isn't so. Of course feedback is important, but so is self-belief, belief in one's musical message, one's musical language, one's unique style. It is such self-belief that will make you persist in your unique musical path and to try to pursue it and to allow yourself to develop on that path despite the pressures to veer from it and to conform to the accepted and clichéed norm.