A patient seems withdrawn and is hard to read. You want to check specifically for depersonalisation without leading them into an answer. Before revealing the answer, decide what you would actually ask.

Start in ordinary language before offering the clinical term: "Have you ever felt as though you're not quite real, or as if you're watching yourself from outside your body?" Then check that insight is retained: "Do you still know that you exist and that this is you, even though it feels strange?" Finally, distinguish it from a nihilistic delusion by asking plainly whether they believe they've actually stopped existing, or whether it simply feels that way.