If you hand a broken calculator to someone who knows how to do math, and they entered 123 + 765 which produced an answer of 6789; they should instantly know something is wrong. Hand that calculator to someone who never understood what the tool actually did but just accepted whatever answer appeared; and they would likely think the answer was totally reasonable.

Catching an LLM hallucinating often takes a basic understanding of what the answer should look like before asking the question.