Explanation: In a statement form, this is agreeing with someone. In a question form, it can be a sarcastic remark related to hypocrisy. For example, a person who is fat tells a less fat person, A: "You need to go on a diet." B: "You're telling me?"
You're telling me, that book was one of the best I've ever read.
You're telling me, I already knew that was going to happen if he did not listen.
You're telling me, if she keeps it up she will lose 20 pounds.
A.
She sure is a good teacher.
B.
You're telling me! She taught me everything I know.
A.
That girl sure is beautiful.
B.
You're telling me. She is the hottest girl in our school!