For me , the easiest way to identify a quality frame from low-end when you dont know brands and labels, is to take a quick glance at the rear dropouts. If they are plain stamped steel, it's low-end. If they are moulded (investment cast), with changes in thicknes, then its probably a quality bike.
Even bikes with stamped dropouts can make rideable beaters.

I recently say a reasonable frame in a dumpster, but on closer indpection it was crash-damaged, with the rear dropouts crushed in.