How does Holden Caulfield express truth throughout the book?

I think we need to be clear that Holden's adolescent angst ridden, and often hypocritical, take on truth is seen through his personal filter. Holden labels almost everyone a “phony,” excepting Phoebe, Allie, and himself. In Holden's eyes, a “phony” is someone who embraces the world’s mundane demands and tries to make something out of nothing—that is, just about everyone who studies in school or who puts on airs in order to do a job or achieve a goal. The fact that no one is acknowledging how trivial and fleeting life is, compared with the grand things we tell one another about reality—how difficult it is to truly love and share oneself with people knowing that all, like Allie, will eventually die—causes him to burn with frustration, even rage. Holden understands on some level one of the most profound truths of mortal life: the superficial matters little because it will not last, yet it is made to seem so much more important. Meanwhile, all around him, he must watch superficial people win honors through their artifice. He thus holds his deepest contempt for those who succeed as phonies: Stradlater, the Headmaster, and all the boys who treat school as if it is a club to be ruled by Social Darwinism. All Holden wants is some authentic living, to hold on to someone like Phoebe or Allie who knows nothing of the world’s superficiality and therefore is not tainted by it, but he is afraid to make it too real out of the justified fear of one day losing them forever.