dave2008
Legend
That was a question about Dispel Magic, I did read that. I just didn't make the connection that you did.There was a question of "how do you know when something is magical" and the answer was "it will tell you." Neither breath weapons or most attacks from monsters say they are magical in the MM.
However, think that is specific to what Dispel Magic affects. There are many things that are not noted as magical (dragon and beholder flight for example), but they are magical (during 2014 discussions WotC called this innate or background magic IIRC). However, that type of "magic" can't be dispelled and therefore is not listed as magical.