Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Frederic Henry tells it all, and doesn’t spare himself. He confesses the constant drinking, the prostitutes, the lies, the desperate thoughts. In the scene where he and Aymo shoot the soldier during the retreat, isn’t he confessing to some kind of murder? And what about at the beginning of the novel when he confesses that, at first, he only pretends to love Catherine and is just playing a game with her? And what about that scene with the baby? Even though it seems like an unreliable narrative (Check out "What’s Up With the Ending?" for more on this), Frederic is perhaps telling his darkest moment – the moment when he felt nothing for his dead or dying lovechild. As if we had any doubts, this guy is not lying to make himself look good. And this makes us trust him when he describes how loving he was with Catherine and with his brothers in arms.
This presents both a compliment and a contrast to his dedication to detailed accuracy when narrating the factual elements of World War I. Here he takes no liberties, and leaves no actual historical fact unverified. He has allegiance to no country, but all the allegiance in the world to humankind, and thus no reason to spin the facts. Frederic’s insistence on historical truth-telling helps us trust that his beautiful and terrible confession is, at least, very close to the truth.