How do you say are you dumb in Japanese?

Something such as "duh" does not exist in Japanese to my knowledge, though if you wanted to show that something didn't need explanation or something similar 分かってるよ (wakatteru yo) works. You could also say 当たり前じゃないか (atarimaejanai ka), which is a sort of sarcastic phrase similar to "You couldn't get it before?"