The way I understand it, an LLM response is a chain of tokens where each is the most probable token. Maybe there exists more complicated candidate and selection approaches than that, but biggest number works for me. For the sake of simplicity, let's just say tokens are words. You'd have access to the probability of each word in the ordering of the sentence, but I'm not sure how that would then be used to evaluate to the probability of the sentence itself or its truthiness.