On Mozart being a genius

Long story short: How do we know Mozart is a music genius, or did the world set his works as the standard and by definition he defaults into being a genius?

A discussion came up between friends on art appreciation and one raised an issue as above.

Art is subjective. I think his works are beautiful, but that does not objectively place him as having produced something good. Or being a genius. How do I know his works are beautiful? Or was I taught to recognise that as beautiful or good? How can we tell these cases apart? Could it be that we are taught to make noise as similar to the classical period and if we don't do it close enough, we are "not good"? At the root, how do we know what is "good"? What's the yardstick here and who put it there?

Imagine, in the past a heavy metal guy composes something, and the classical world disses on him. He could have been a genius in the heavy metal world, but he is regarded as noise at that period.

Has the world used his standard as The Standard and everyone else who tries naturally pales in comparison to him? Or is there an objective way to show that these composers are geniuses?

I know this is not a music theory question, but I need to call upon the music world to put up a world class argument.