I use to determine the butt edges by first cleaning out the tube's ID with a rag and WD40. Then holding the tube up to a light and eyeing along the tube's ID look for the lines of reflection to take a slight jog. This is where the surface angle changes (the butt starts or stops. Using a long thin indicator (spoke) one can insert it into the ID and watch for the end to match up with the reflection's change. Remove indicator and align along the OD to mark that point. Repeat for the other butt changes. This method can work well with a clean tube ID and care. But a gage makes things so much easier.

I have purchased tube that were not what I was tole they were. In that case the seller was a privet hobby builder who had bought a stash from another builder. Turned out what he thought was all Columbus Genius turned out to include a couple of stainless steel tubes. Found out the hard way when trying to fillet the seat tube to the BB shell. Andy