Modern metal is based on classical note progressions and chords, and sounds quite different to the 70’s-80’s metal. Those were more often than not just rock music (based on blues*) that was more exciting or had more controversial lyrics in it. Nonetheless, it was all branded as “heavy metal.” It was all “metal,” like many 80’s hair bands that really were just rocking out but had scandalous outfits or songs with rebellious natures. Twisted Sister is not metal today. Black Sabbath, however, is.
Back in Black by AC/DC is not metal by today’s standards, it would be classified as Hard Rock, for instance. By comparison, Judas Priest’s Painkiller is very much Metal by today’s standards. Listen to the way the music flows, the note changes being used, the harmonics, and you’ll find that eventually you can generally identify rock from metal pretty easily.
Note: this is for general distinctions and not sub-genres. That’s too complex of a topic to cover in this comment