Guy Gardener wrote:
You're going to have to cite or quote from an episode for me to believe you.
I'm not being an asshole, that's just how it's done.
"At the Rob Benedict Panel at a Jus in Bello con, Rob confirmed that Chuck is in fact God. He explains that other members of the cast, as well as Eric Kripke himself, had asked him how it felt to play God." --Video
And for an actual in-show quote (which is misleading, but still a quote): "Well, there's only one explanation. Obviously I'm a god... I'm definitely a god. A cruel, cruel, capricious god." --Chuck, The Monster at the End of the Book.
Joshua (the black angel in Heaven's garden) also told the boys that God was on Earth and, despite what Castiel said about Dean's amulet, that they wouldn't be able to find him if he didn't want to be found. He also appears dressed immaculately in white after narrating the story (in which he groans about how hard it is to end a story just right) in the season five finale before disappearing. Plus all the other hints scattered through his appearances, such as the ad for Mary Magdalene as an S&M mistress on his desk.