Shéi zhīdào？

谁知道?

Who knows?


Notes:

谁(shéi): who

知道(zhīdào): to know

Example:

A:明天有聚会吗？

   Mínɡtiān yǒu jùhuì mɑ？

   Is there a party tomorrow?

B:谁知道？

   Shéi zhīdào？

   Who knows?