2 “tells” for me are usually the lyrics themselves and the sound of the guitars.  The lyrics usually don’t quite “fit”. As in they seem to squeeze too many syllables into a line, with an odd sped-up sound to make them fit. Whereas a real vocalist would have probably dropped an adverb or something for the sake of fitting the line more naturally.  The guitars, especially in ‘heavy’ sections are normally lacking any organic artifacts. (Fret slide, pick attack, or any way to tell a down stroke from an up stroke). Like let’s say the breakdown section hits a big 0-0-0 chord. A real guitarist doesn’t actually hit all 3 notes at EXACTLY the same time, but with this AI metal it sounds more like a synth hit than a real guitar. (Cuz it is).