[WS:] If you are of an Aristotelian persuasion, one possible answer is: art is the potential embedded in the material that is actualized by the work of an artist (e.g. a sculptor chiseling out an aesthetically pleasing shape out of stone), and then by perceptions of the audience (e.g. spectators perceiving an object created and cognitively reconstructing the aesthetic qualities embedded in it by the artist.)
If the potential is not there (e.g. a pebble may resemble a seed but does not have the potential of becoming a tree), or if it is there but it is not actualized properly (e.g. a seed that dies or grows to a sickly tree due to gardener's ineptitude) - it is not art.
How can you tell what is and what is not art? Thanks to apriori cognitive categories that allow you to recognize aesthetic qualities. In the same manner people know that there is a difference between right and wrong, and cause/effect relationship and mere coincidence, albeit application of these apriori principles to the experiential world is far from unambiguous, uniform, and automatic.
If you are a relativist, however, the answer to this question is whatever people with influence and authority, or pretending to have influence and authority, say it is. Anything goes - no skill or effort of any kind is necessary. Virtuoso performance or charlatanry, they all can have any value, that is, no particular value at all.
With that in mind, I do not think anyone but flippant relativists and poseurs would in good faith take fart for art.
Wojtek