  The ability to predict an NLP model's accuracy on unseen, potentially
out-of-distribution data is a prerequisite for trustworthiness. We present a
novel model that establishes upper and lower bounds on the accuracy, without
requiring gold labels for the unseen data. We achieve this by training a
discriminator which predicts whether the output of a given sequence-to-sequence
model is correct or not. We show across a variety of tagging, parsing, and
semantic parsing tasks that the gold accuracy is reliably between the predicted
upper and lower bounds, and that these bounds are remarkably close together.
