... is surprisingly easy, because there is something available which is the opposite. This is called science and is structured into subjects like mathematics, physics, biology and chemistry. The idea behind science is to use the rational mind for recognizing the nature. The good news is, that everything in the world fits in one of these categories. It is either art or science. In general, art is creating something from scratch which was not there and results into a cultural history, while science is per default true and bans everything what is not science. The best example to make the difference clear is a perpetuum mobile. From the perspective of physics it is a failure and equal to non-science. It is something which violates the laws of nature and people who are trying to build such a machine are pseudo-scientists. The same device (a perpetuum mobile) is from the perspective of art a valid contribution. That means, it is possible to paint such a machine in oil and it is possible to make a video about it (which is fake, of course).