Kurt, IMO, my answer to your question as to whether there is a definitive way of determining plain gauge from butted is yes and no (how's that for definitive!). Almost daily I tap (w/fingernails) frame tubes on several of my unbuilt frames just to hear them ring (I'm strange that way). There is a definite sudden difference in timbre or tone (unsure of the difference) as the tap goes over a butted area. This sound is different than the gradual change in tone one hears as the tap appoaches the end of tube of both butted and plain gauge tubes.
I say no in regards to heavy gauge tubing or when brake cables etc are attached to tube. HG tubing is so thick it doesn't have much of a ring (and similar riding quality IMO) and butting is doubtful in any case. Cable(s) attached to tube mutes the ring and makes telling the difference much harder if not impossible. I don't have enough experience with quad-butted or any other tubing schemes to be able to tell.
I seem to remember others saying this method is unreliable and perhaps it is. However, I hear the difference when I play my musical frames!
Jack Romans
Sacramento, Calif