Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
While O'Brien's tone in regards to the war jumps around from emotional to clinical to manipulative to epic, his tone in regards to storytelling and the truth is constant: He is absolutely familiar and close to his subject. He tells his stories with the complete confidence that he is right, that he understands his subject inside and out:
The town could not talk, and would not listen. "How'd you like to hear about the war?" he might have asked, but the place could only blink and shrug. (Speaking of Courage.32)
Even when he shifts reality from under our feet—or uses oblique phrases like a town "blinking and shrugging" coupled with casual language like "How'd you like to hear about the war?"—we never doubt that he knows what he's doing, and that his manipulations and language choices serve a purpose.