I would take a look at Randy Jawa's My Ten Speeds site, but in general, I would look at the frame/fork first.

1) Forged rather than stamped dropouts
2) Tubing type
3) Lugged (generally)
4) Level of workmanship on the frame. Do you perceive quality construction?
5) Weight. A good steel lugged frame will generally be 5 lbs or so, including the fork (but not the headset or any other parts).

And to add to what has been said, generally bikes with stem shifters indicate a lower pedigree.