How to determine the key of a song
This has bugged me since the big bang. I’ve just come up with a method that seems to work really well for me.
This solution will only work for keyboard players. The basis for my method is the assumption that all the notes in the key that the song is in, will sound fine when played alongside the piece.
All I do is transpose my keyboard a semitone at a time and then run my finger across the white keys on the keyboard. If the gliss sounds good, then chances are that you are in the right key if it doesn’t sit very well then transpose and gliss again. Once I find a transpose setting that works, I can then determine whether the piece is major/minor based on its mood or just by noodling on the major/minor scales to see which fits better.
If you have an EXB radias expansion or any synth with a mod sequencer, then there is an even easier approach. What you can do is create a radias patch and then create a mod sequence that steps thru the 8 notes of a scale. In my case, I really only care about major and minor scales. Instead of transposing and glissing, you just need to hold done a single key to loop through the notes of a scale rooted in the held key.
It is amazing what you discover once you play along to a song and you are aware which key you are playing in. So much gets demystified because it becomes so much clearer what progression is being used. What you once thought were ingenious melodies get unravelled as mere scalar runs etc.
I think this will perhaps make up for my lack of ear training and will also get me more milage out of the practice I’ve done in C maj/A minor.

this is a major issue for me too…
will try your technique