Most explanations of the rational authority of testimony provide little guidance when evaluating individual pieces of testimony. In practice, however, we are remarkably sensitive to the varying epistemic credentials of testimony: extending trust when it is deserved, and withholding it when it is not. A complete account of the epistemology of testimony should, then, have something to say about when it is that testimony is trustworthy. In the typical case, to judge someone trustworthy requires judging them to be competent and sincere. In this essay we develop an exchange-based account of testimony that shows how those who receive testimony are in a position to evaluate the sincerity of speakers.