I think you should just stick to the old NWT as a sign and then there should be a question regarding a piece of clothing. Something totally off the wall.
"I love your blue shoes" when their shoes are obviously brown.
"Nice bow tie." When they are wearing a standard neck tie.
Something that is not really standard, but would be obvious to someone in the know. It could be laughed off if it's a mistaken identification, but answered with another similar remark if the person is a fader.