I think that this debate can be a fairly intense one. In my mind, the most evident reason why Holden can be considered not sane would be the setting of the narrative. Holden is narrating the story from a place where he requires some level of regrouping:
Holden, the narrator, is telling the story from a place in California, near Hollywood. Because he is run down physically, and is probably mentally exhausted as well, it appears that he is in a sanitarium to recover and regain his strength.
Adding to this would be the fact that at the end of the narrative, Holden "still does not understand all that has happened to him" would also add to the fact that he is a bit unstable. Finally, the fact that Holden demonstrates an inability to fully function in the world around him might help to prove while he can be considered someone in need of some level of professional help. Yet, all of these can be argued that these would be reasons why Holden is more "normal" than the rest of the world. While he cannot function in the world, this might be due to the fact that the world is out of balance, and not Holden. The people that Holden interacts with for the most part are not authentic and duplicitous. While we might dismiss this as "the way things are," I think that there is an argument to be made that Holden seeks to appropriate the world as it should be and not for what it is. This is a testament to Holden's high sense of moral standards, and not insanity.