I think it takes a trained ear. If ever you can hear a song, and remember the EXACT key it's sang in...(Say for instance you can hum a song in the exact key that it's in....) I believe it's the hearers' EAR. I may be wrong though. I don't have any formal training or any piano skills really...but I just KNOW when something is being sang in the right key and when it's not...kinda' confusing, eh?? God bless.