easiest way to tell a decent frame (and yes, there are exceptions):

Look at the rear dropouts. Nicer frames have forged ones, while frames to stay away from usually have cheap stamped steel dropouts.

I would have never noticed my '63 Schwinn Supersport as something worthwhile until I looked at the dropouts. Turns out what I thout was a crappy bike-boom schwinn was actually a hand fillet braised psudo-paramount