You cannot hear a hallucination. You can only notice where one is likely: unsolicited specifics, suspiciously smooth answers to obscure questions, and claims you have no way to check.
Specifics you did not ask for
Names, dates, numbers, quotations, and citations are the parts of an answer the model had to generate from patterns rather than retrieve from a record. They are also the parts that feel most authoritative. That combination — high risk, high trust — is why they are the first thing to check.
Smooth answers to thin questions
When a topic is thinly covered, the patterns are weak and the model fills the hole. The result often reads more cleanly than the underlying knowledge deserves. If a question about something obscure, recent, or local comes back as polished as a question about a famous topic, the polish is the clue.
Hard-to-check claims
A source you cannot open, a study you cannot find, or a role you cannot confirm does not prove the answer is wrong. It does mean you cannot tell the difference between a real detail and an invented one, so the claim stays unverified until something outside the chat confirms it.