How do you know that you are sure that your parent are your parents?

Your question asks about your knowledge of your own certainty: "How do you know that you are sure...?" That's a question about your knowledge of your own mind rather than about your knowledge of your parentage. So I think the best answer is "By introspection -- by looking inward -- and perhaps also by examining your own behavior for signs that you're unsure that the people you take to be your parents are in fact your parents."

If, instead, you meant to ask how you can be sure that the people you take to be your parents aren't impostors, then you might have to seek DNA evidence (in the case of biological parents) or adoption records and other documents (in the case of adoptive parents).

There's also a facetious answer: "Of course your parents are your parents -- that's a tautology!"

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