Usually the easiest place to see quality is in the rear dropouts (where the wheel is held).
Next time you are in a bike shop, check out the detail on the dropouts in different price ranges.
The seatpost clamp is another good sign.
The really important things, like alignment and facing of the tubes is very difficult to see, and often overlooked by bike tests. Its really easy to make a cheap frame which looks good but is badly finished inside.