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?」