One might consider using a metric based on language model probabilities for sentences: in eval-uating a language model on (already existing) test data, a higher probability for a sentence (and lower perplexity over a whole test corpus) indicates better language modelling; perhaps a higher probability might indicate a better sentence.
here we are looking at generated sentences, which have been generated using their own language model, rather than human-authored sentences already existing in a test corpus; and so it is not obvious what language model would be an objective assessment of sentence naturalness.