As epistemic and normative learners, children are dependent on their developing skills for evaluating others’ claims. This competence seems particularly important in the current digital age in which children need to discern valid from invalid assertions about the world in both real-life and virtual interactions to ultimately gather and accumulate robust knowledge. We investigated whether younger and older preschoolers (N = 48) understand that a speaker's knowledge claim (‘I know where X is’) may be correct or incorrect given objectively accessible information (about whether the speaker had perceptual access to a critical event). We found that both younger and older preschoolers accepted correct knowledge claims that matched observable reality, but that only older preschoolers reliably rejected incorrect knowledge claims that did not match reality (the speaker lacked perceptual access). Nevertheless, a considerable proportion of younger preschoolers both rejected incorrect knowledge claims and gave valid explanations, suggesting that the ability to scrutinize epistemic claims develops gradually from around 3 to 4 years of age. These findings may help integrate research on children's norm and theory of mind development.