Answer
If I asked you ‘Are you a truth-teller?’, would you say ‘yes’?
Work Step by Step
You could ask the person, "If I were to ask you if you are a truth teller, would you say yes?"
If the person is a truth teller, they would truthfully answer either "yes" or "no" to the question. If they are a liar, they would lie about their response to the hypothetical question, thus giving the opposite answer of what a truth teller would give. This way, you can determine whether the person is a truth teller or a liar based on their response.
