actually, one way is to do perfect pitch training...

basically, just listen REALLLLY closely and just soak your ears into ONE note.... feel the note vibrate... feel it harmonize within you...

then move to another note.....


if you play the A string, and then play any other note on the guitar, and guess which note you played, that's REALLY good for your ears...

if you play an A chord, and then play any other note, and guess it.. that's also good...

if you try to play a random triad or interval pair (harmonized, played at the same time)... and then try to go in there and figure out what pitches are being played... that's also good.... then, singing the two pitches of course to make sure they're the right ones... (does this make sense?)