Nǐ quèdìng ma?

你确定吗?

Are you sure?

Notes:

你(Nǐ): you

确定(quèdìng): be sure

吗(ma): a question word in order to turn a statement into a question

Example：

A: 听说John辞职了!

    Tīnɡshuō John cízhí le!

    I heard that John have resigned!

B: 真的?你确定吗?

     Zhēnde? Nǐ quèdìnɡ mɑ?

     Really? Are you sure?