Journeyman Bike Commuter
Join Date: Aug 2009
Location: Alexandria, VA
Bikes: '88 Fuji Saratoga, '12 Jamis Sputnik, '13 Motobecane Fantom29 HT, '16 Motobecane Turino Pro Disc, '16 Motobecane Gran Premio Elite
Mentioned: 4 Post(s)
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Quoted: 65 Post(s)
Quality of the paint job or tubing decals are an initial tip-off, but they may become marred over time.
I also look at the finishing work done before the painting, filing and cleaning around the brazed junctions. Obviously more fancy, cut-out lugs are an indicator. As are the finished ends of the seat stays and their junction at the seatpost.
Just tapping the tubing with your fingernail and listening to the "ring" can tell you something, too.
As for forks, look at the dropouts. How well finished is the dropout to fork end interface? Is there a sloping crown or flat crown. Details on the crown and cutouts? If it's a unicrown, are there any indicators of tubing. Sometimes the steerer tube has stamped markings of the tubing type, but you have to disassemble to see those.