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Tell me something only you would know.
(comicpress.socksandpuppets.com)
These situations are a great opportunity to ask your party members questions that they normally refuse to tell you about.
Yeah, you could just ask questions that you wanted to know the answers to, then go off vibes, or you could write a list of questions that only you and the person knew the answers to, and have someone else ask them, write the answers and bring them back to you.