i've thought about this too!
what i've just started to do is to play an open string repeatedly (2 notes a second or so) with my pick above the 17th fret all with down strokes. i'll do this for 10 seconds or so and listen to the sound as closely as possible. then i'lll move the pick above the 18th fret and do the same thing. then i'll move to the 19th fret and do the same thing. and i keep moving the pick a half an inch or so towards the bridge still doing the same thing until my pick hits the bridge.
it seems the more you do this the more you notice the change in sound with each movement of the pick.
then i guess you could do the same thing but with alternate picking as well.
if you really want to get hard core about it you could do the same thing for each fret of every string rather than doing the pick movement thing with just an open string. did that make sense?(i'm tired)
i have not done that last option above yet.