What do you mean by when someone "questions" a fact about yourself? Like if they say, "Is that your real eye color?" or "You can't really be a size 6?!"
In ANY case, I would imagine, if a person questions anything about you that you know to be factual and true, you can either answer them politely by telling them, "Yes, as a matter of fact, that is true," or you can be atrociously rude and say, "What's your problem? You calling me a liar?"
Irregardless...at the root of it all, why give a fig what someone else thinks anyway? :)