How should I know?
I don't know; I'm not the person who can answer that question. Often expresses the speaker's annoyance or frustration. A: "Is Nick coming tonight?" B: "How should I know? He hardly talks to me anymore." A: "Whoa, what happened in here?" B: "How should I know, I just got home myself."
How should I know?
and Don't ask me.Inf. I do not know.; Why should I be expected to know? (Shows impatience or rudeness.) Bill: Why is the orea called the killer whale? Mary: How should I know? Sally: Where did I leave my glasses? Tom: Don't ask me.