In a sense, "Is THAT what you're wearing?" can only be answered in the affirmative: "Yes, this clothing is what I HAVE ON at this moment!" By contrast, it's said that "Are you asleep?" can only be answered in the negative. Generally, though, our answers are not a simple negative or positive.
Steve will ask me, "Is THAT what you're wearing?" to call attention to the fact that I'm not ready to go somewhere. He's likely to BE ready, and I'm likely to be wearing my nightgown or gardening attire caked with mud. In this case, my answer would be "No!" or "Just hang on a few minutes."
I tend to ask Steve, "Is THAT what you're wearing?" to call attention to a color-coordination* problem or other wardrobe malfunction,** usually when he's getting ready for a rehearsal or performance. In this case, I make sure he's presentable before he leaves.
When my father, Erven, would leave to go grocery shopping, my mother, Charlotte, would often say, "Don't pinch the tomatoes!" Married couples need ways of gently checking up on each other. It helps make our worlds go 'round.
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* Color coordination seems to be more difficult for men than for women. Geneticists see this as a chromosome issue, but Steve's brother Phil attributes it to the fact that little boys are typically given the eight-color box of crayons, while little girls tend to be given a higher multiple of the basic eight. Today's mega-box holds ninety-six colors.