OPINIONS
 

Is agreeing or disagreeing?

A: He talks too much.

B: You're telling me! I sat next to him on the bus yesterday!

A: It's hot today, isn't it?

B: You can say that again.

A: He's an intelligent boy; the real problem is that he's lazy.

B: I think you've hit the nail on the head.
  You're telling me.
I know that

You can say that again.
I agree strongly

hit the nail on the head
describe
something
accurately