I was thinking that the tonic chord provides a big clue. You wouldnt tune to open D, but then play in the key of G# or something (if you're in open D, your 1chord will probably be D). But once you throw in capos to the mix the tonic chord clue isn't very helpful (you can tune to open D, but then put your capo on the 2nd fret to play in E, etc)... Also, side question, how does using a lot of different tunings work live? Like were 50% of joni mitchell concerts just her tuning her guitar a million times?
multiple guitars. depending on your income bracket you may even have a person hand them to you and take away the previous one