By the attitudes of some who claim to love his music.
As Adorno decreed, the job of a composer was to write music that would repel, shock, and be the vehicle for 'unmitigated cruelty.'"
[quotation in The Rest is Noise by Alex Ross, referring to the Darmstadt and Cologne schools of the late 1950s and early 1960s.
So, my contention, his music was not formed to be liked, admired, or appreciated, but to be merely music--hopefully music that would elicit a gut level reaction. If so, then my view of it would far more please the theory of the school than that of purported admirers. However, we must keep in mind, that despite the theory, everyone wants to be loved, it's just that sometimes we want to exclude the "rabble" from that warm embrace. If so, more's the pity, because it is in that rabble and their acceptance that any chance of a lasting contribution remains.
I become convinced that in atonal and serial music after the advent (partiuclarly in serialist music), what is really being conveyed is the composer's inability to compose without a method. Just as, after Picasso and their crew, as we move into the realm of abstract expressionism, what is really being revealed is the artists' inability to deal with any classical form. So, instead, like Pierre Boulez, we construct musical theory in which detraction is far more important than putting forward any coherent sense of what it is you are about. "I can't compose a concerto or a sonata, so I'll crush them instead."
Interesting musical theory.