Children are sensitive to their own and others' epistemic states and use these to guide their learning and communication. Here, we systematically examined children's use of epistemic states to make diagnostic social inferences. Specifically, we investigated children's group membership inferences based on what others do and do not know and what role children's own knowledge states and the type of knowledge play in these inferences. Across two preregistered studies, 7- and 8-year-old children (N = 100) were shown targets who were knowledgeable (Study 1) or ignorant (Study 2) of familiar and unfamiliar knowledge items that were either general or culture-specific and asked to guess whether the targets would be linguistic ingroup or outgroup members. Children inferred that the targets would speak their native language if they shared targets' epistemic states and a foreign language if they did not. Importantly, these patterns were particularly evident when epistemic states involved cultural knowledge. Further, children's inferences became more nuanced with age. These findings suggest that others' knowledge states are socially meaningful in childhood and children use their own and others' epistemic states in flexible ways to guide their diagnostic social judgments.