Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third-Person (Limited Omniscient)
We know from the get-go that the narrator of this story is third-person because s/he doesn't have any direct involvement in the plot of the book itself.
Instead, this narrator remains at a distance and starts the book by telling us,
Amory Blaine inherited from his mother every trait, except the stray inexpressible few, that made him worthwhile. (1.1.1)
But make no mistake. This narrator might be third-person omniscient, but this is very much Amory's story. We can tell that there are limitations to the narrator's omniscience because, as the novel plays out, we aren't given access to anyone's thoughts except Amory's. But, hey, that's pretty appropriate—Amory doesn't care about anyone's thoughts but Amory's, either.