There's this guy on youtube that I'd watch, he has a channel called 'Jazz Theory'. Anyway, one profound thing I heard him say is that you're not playing by ear until you can hear the numbers in anything you listen to.
And by numbers, what chord number is playing (I-VII), what melody number is playing (1-7). So as you're listening to music you constantly hear the different numbers being played. I think this is very useful.
So as an example, if I take the chords of the major scale. There's 3 major chords (I, IV, V) and 3 minor chords (ii, iii, vi) and one diminished. The diminished chord isn't used as much as the others. The root chord is easiest to hear as everything resolves to it. And it's relatively simple to hear if a chord is major or minor. However, sometimes it's hard to distinguish between the ii and iii, and the major chords IV and V. Although the V is easier as it sounds like it wants to resolve to the I.
I have a harp and sometimes I'll just randomly pick a chord or note without looking and try to figure out what number it is within my scale.
I'm still practicing this and I'm trying to think of ways to be better at this. But I was wondering, for those of your that can hear numbers. Do you have any tips or exercises on how you did it?