Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
The author's attitude toward the subject material is made evident by her decision to use the form of a memoir (albeit it a fictional one). Jim's memoir is nostalgic and is dominated by a romanticized, rose-colored vision of the past, his youth, and the American West. He wistfully views the past as something beautiful and unrecoverable. The lengthy descriptions of the landscape provide great insight into the authorial tone. There is both a sense of beauty and of loss and longing in these passages.
We can also assume that the tone of the novel is shaped by Cather's own attitude toward the events of her past. Cather, just like Jim, moved to Nebraska when she was ten and grew up on the prairie. She lived in a town called Red Cloud on which the fictional Black Hawk is based. Because of these similarities, we imagine that Cather's own nostalgia is what is responsible for the tone of the book. It takes on the quality of her memoir, not just Jim's.