The ability to monitor uncertainty is critical for self-directed learning. However, little is known about children's ability to detect uncertainty in their mental representations. Here we asked whether a spontaneous information-gathering behavior -- social referencing -- reflects uncertainty during early childhood. Children ages 2-5 were presented with one or two objects, heard a label, and were asked to put the object in a bucket. In Experiment 1, there were one or two novel or familiar objects, with the referent being ambiguous on two-novel trials. In Experiment 2, there were additionally trials with one familiar and one novel object. In both experiments, children referenced the experimenter more when the referent was ambiguous. In Experiment 2, children also referenced the experimenter more when there were one familiar and one novel object, but only when the experimenter did not gaze at the target, suggesting that children's social referencing is sensitive to epistemic uncertainty.