When I was in school, the music teach taught us thus: There are innumerable sub-genres of metal — heavy metal, black metal, death metal, what-not. You can usually tell them apart by what size amps they have.
Of course, he was being sarcastic — it's not really that easy. In truth, it's more a combination of how many distortion pedals they have, how bad their "singer" is — Cradle of Filth and Subway to Sally come to mind as bands that are lethal unless played through a karaoke system —, and how inane their lyrics are (Blind Guardian are a good example here, as are, once again, Subway to Sally).
"We are gathered here tonight,
to lay to rest Gwenhwyfar La Fey,
whom we now know was first born dead
on the seventh day of July, 1777.
Abigail must be nailed to her coffin with seven silver spikes,
one through each arm, hand, and knee,
and let the last of the seven be drawn through her mouth
so that she may never arise and cause evil again."