Hello folks just thought I'd make a rare appearance. There are many ways to determine the quality of the tubing, though not the maker beyond what Dale said about tube markings. Be aware that if a frame was repainted in many cases these marking have been eliminated. If a bike is built up I first check the dropouts. If the DO's are cast this usually means quality tubing. If they're stamped it gets a bit more difficult and it becomes a judgement call. Next I tap the tubes with a fingernail, this will show you where the butts are from the change in sound. This is a bit tricky too since the closer you get to the joint on even a straight-guage tube the sound will chance, but with a butted tube it's a more abrupt sound chance.
If it's a bare frame nothing works better than a dentist's mirror and a penlight to look inside the tube. Look also at the miter of the tubes since quality miters come on quality tubes. Also English and Italian frames look different, at least to my eye, though that can't be said about bikes from anywhere else. I'd tell anyone not to worry about the maker of the tubing, but the quality. If you can isolate who the builder is-- which is much more important than the tubing-- you can make an educated guess to the tubing. good luck. enjoy, Brandon"monkeyman"Ives Going for a ride in Sunny Santa Barbara than to go build bikes at a friends shop.