Well I think you answer the question with this article. But, off course the definition of an art movement is always blurry because it depends on the opinion of the viewer and also on the artist purpose. If an artist sets out to create a piece is he really thinking about the "rules" of a movement or is he simply creating art? I believe the source of the confusion is the manny different opinions that you can find about the same work. I do think it's possible to tell them appart but does it really matter?? Art is to be absorved, enjoyed and not to be labeled and standardized. Instead of asking yourself if it's Art Nouveau or Art Deco or Modern Art or whatever definition the critics invent, just ask how it makes you feel and what it tells you. Once a person gets the thinking trapped in rules he or she won't be able to appreciate it for what it is but only for what others think of it.