How can you/we know?
Whatever we may mean by “art” (and I think there are several facets of the term that we need to discern between), there is also a fundamental question whether it springs out of the creator’s intentions, some more objectivised evaluation of what is presented how, or from the experience elicited within the viewer which is again quite dependent on context.
If the viewer somehow plays a part, is there then anything we can state about what is art with certainty/generality?
EDIT: @lphilpot: Thanks for the quote! I shall add it to my collection of photo quotes. 