BSO identification for amateurs
It used to be easier to tell BSOs from "real" bikes, but now the technology is changing so rapidly, and it's much harder.

On a bike older than about 20 years, one would first look at the frame, in two places:


The joints.  Better quality frames would be "lugged", with the steel tubes brazed into fittings (called lugs) that secured the joints.  Cheap frames would just have the tubes welded together.  (Welding, as used then, would destroy the thin, high-strength steel used in better frames.)
The dropouts.  Cheap frames would have rear dropouts that were quite clearly just stamped from sheet metal, while the front dropouts would be similar, or would simply be slots cut in the ends of the flattened fork tubes.  Better quality would use forged dropouts.


These days the dropouts are still a tell, but not nearly as reliable as before.  And frames are often made of materials other than steel, and even steel can now be welded, using a rapid-weld process originally developed for aluminum frames.  (Though a good-quality lugged steel frame is still a thing of beauty.)

Other than the frame, the most obvious identifier of low quality is a one-piece crank.  This was true 20 years ago and is still true.

Well, I'll take that back -- one more thing:  Twist shifters are highly suspect.  They are cheap and have an extraordinarily high failure rate.  There presumably are some good quality twist shifters, but they're the exception.

Make that two more things:  With a multi-speed bottom bracket, check the material used to make the chainrings.  Very often BSOs have steel rather than aluminum chainrings.  A better quality bike may have a steel "granny" ring, but the others would be aluminum.