On a good quality bike look for a tubeset label. It will be found on most bikes unless they have been repainted. Good frames will have labels such as Reynolds 520, 531 or higher numbers and the label will indicate that the tubes are butted. Columbus and True Temper are other good brands but quite a few other makers of good steel tubing produced seamless butted frame tubing too. Other things that may be on tubing labels are Chrome Moly or Manganese Moly material listings.

Cheap bikes may have tubing labels too but it will typically say High Carbon, High Tensile or have a number such as 10XX steel. Stamped sheet steel rear dropouts, without an integrated derailleur hanger, are a sign of cheap bikes as are the rear dropouts being installed by crimping and spot welding the frame tubes.

Look at quite a few bike frames and you will soon be able to see the differences between a good quality frame and a lower quality one. Look at the components fitted too as things like cranksets with non removable chainrings are definite signs of a low quality bike.