You are telling me! = 那還用說！

意思:
1. used to emphasize that one is already well aware of something
    那還用說！ 

2. used to say that you strongly agree with what someone has just said 

    亦可解作 「你說的太對了！」

例子:

"Stephen's in such a bad mood today." 「斯蒂芬今天心情很不好。」
 "You're telling me!" 「那還用說！」

You're telling me! 在這裡的意思是「我早就知道，不用告訴我。這還用說嗎?」