Carl Sagan's questions for aliens. (1996)

"The Demon Haunted World" (1996) Chapter six has this passage "Occasionally, I get a letter from someone who is in 'contact' with extraterrestrials. I am invited to 'ask them anything.' And so over the years I've prepared a little list of questions. The extraterrestrials are very advanced, remember. So I ask things like, 'Please provide a proof of Fermat's Last Theorem.' Or the Goldbach Conjecture. And then I have to explain what those are, because extraterrestrials will not call it Fermat's Last Theorem. So I write the simple equation with the exponents. I never get an answer.

So, what question would you ask, mathematical or other, to determine if the aliens are more advanced than humans?

Or, what question would be most useful to know the answer to?