[edsrzf]

Maybe something like "Do you have more older siblings than younger siblings?" would work. One twin has to be born before the other.

That question has some problems, but I feel like it could serve as the basis for a more specific question that would define how an only child or middle child would answer without introducing much bias. It would also have to define how half-siblings are counted and probably some other things.

[RogerL]

So you are going to burn one entire question that is entirely worthless for the vast majority of Chinese that have been born recently?

[Intermernet]

Interesting read: "One Child Policy and Arising of Man-Made Twins". http://paa2013.princeton.edu/papers/130113

[pavel_lishin]

It's entirely possible that the twins would not know which one was born first.

[daleharvey]

Its entirely possible the person could not know the answer to any of the questions, I dont think that really counts