"I know it when I see it" is the standard line for deciding if it's art, or if it's crap. Of course, that line is in different places for different people:

Three German refuse collectors are to be sent on an Art Appreciation Course after accidentally taking down and incinerating a sculpture in Frankfurt, believing it was rubbish.
...
The artist conceded that it was possible he had been almost too successful, as his aim was to create something so real, it would not be seen as art.

"Almost" isn't the word for it.