This article lists 5 characteristics that you can use to determine the quality of paintings regardless of the type and quality of the art. The characteristics are:
The author is convinced that anyone can use their own fundamental knowledge to compare artwork based on these five inherent properties. Even if other people have a different opinion than you do about a particular work of art, this article should help you explain how you came to your conclusion.
The article expands on each characteristic to give you a benchmark.
For example his list for Beauty includes
You can read the complete article at http://emptyeasel.com/2006/11/18/how-to-judge-art-five-qualities-you-can-critique/0