Is there a key to being able to find the harmony line in a song and what would that be?

Answer 1

Well, you could generally hear it and/or figure it out, provided you already know music theory. That goes for pop songs and relatively simple pieces; for jazz, you'd have to have absolute hearing and listen many tiumes very carefully. I can, for example, pretty much just start playing along any rock song without thinking, they're all so cliched you just hear the first chord or two andf you know where they will go; from there, you can figure out the line(s) you're after.