In México we use ¿Quien sabe? to express that we don't have the answer and we don't know who has it or if it has an answer.
If it's spelled with a tilde, it's a question. - ¿Quién sabe? - Who knows?
Without the tilde, it's a statement- -quien sabe - who knows.
For instance, - Necesito encontrar a quien sabe la respuesta- I need to find someone who knows the answer.
Try double-clicking on words on this site - you'll get a popup translation. Great for simple questions about individual words!
--¿Dónde queda Madagascar?
--¡Quién sabe! (sepa, no sé, no tengo idea, I don't have a clue, who knows...)