Man, are you for real?

[Jackson6612]

Person A: What is this country called the United States?
Person B: Man, are you for real?

Person B is actually amazed at hearing that A doesn't know about USA - which almost everyone knows about unless one is living under the stone. Does the B's response or his use of the phrase "for real" in this context correct? Please let me know.

[DamianWarS]

saying "Man, are you for real?" is a rhetorical question. Rhetorical questions answer themselves and they are clear what they are talking about usually through the context and tone. Person B is so shocked that his statement brings into question if Person A is real or not. Obviously Person A is real and Person B knows he is real but his statement acts as a sort of rhetorical idiom question where he is basically expressing his amazement towards Person A's lack of knowledge. And you are right that it is used correctly.

But speaking of idioms your use of the idiom "living under the stone" should be "living under a stone" or more commonly "living under a rock". Living under the stone would be speaking about one specific stone instead living under a stone (or rock) is just talking about stones in a general sense which is want the expression is trying to get across.

[Jackson6612]

Damian, thanks a lot. You are really good.