Ok, Larry, I agree that something has to either be perfect or not and can't be "almost perfect" just as something cannot be "rather unique" - either it is or it isn't. What I meant was, as another poster said, ask me to sing a note and if I don't get it spot on, I'll be no more than a semi-tone either way. Likewise if I hear someone playing or singing something, I can probably tell you what key they are in, without looking to see how people are playing their instruments.
As for capoes, if I don't use the open string chord for G, I just know that G1 = Ab, G2 = A, G3 = Bb and so on. Glenn's idea above sounds interesting: try it and let us know how you get on.