Here's how we know:
* First-person perspective: The story is told from the "I" perspective, with the narrator directly experiencing and observing the events.
* Personal thoughts and feelings: The narrator shares his personal thoughts, feelings, and opinions throughout the story. For example, he expresses his admiration for Johnny, his frustration with Darrel, and his fear of the Socs.
* Specific details about Ponyboy: The narrator describes his own appearance, struggles with school, and relationship with his family, providing clear details that identify him as Ponyboy.
* The author's confirmation: S.E. Hinton, the author, has explicitly stated that Ponyboy Curtis is the narrator of the story.
Therefore, based on the narrative style, the details shared, and the author's confirmation, we can confidently conclude that Ponyboy Curtis is the narrator of "The Outsiders."