The ability to monitor epistemic uncertainty is critical for self-directed learning. However, we still know little about young children’s ability to detect uncertainty in their own mental representations. We developed a novel measure to test whether children can assess their lexicon and thus evaluate their own uncertainty about their lexical representations. Children’s ability to monitor their decisions were measured during a classical looking-while-listening procedure. Across two experiments 120 two-year-old children were tested on their knowledge of words that they supposedly know (according to their caretakers) and novel words (taught in the lab). The test was administered by a speaker previously seen as reliable (providing correct labels to known objects) or unreliable (providing incorrect labels). Children accurately monitored their errors when tested on familiar words but not on novel words and were more likely to do so when the speaker was reliable, rather than unreliable, despite displaying similar performance levels on the word recognition task. This suggests that by two years of age, children can reflect upon their own linguistic knowledge to know whether they know or don’t know a word.