linky, This is fun! Are you off your rocker? Are you insane? Do you have a screw loose? Are you soft in the head? Are you demented? Are you mad? Are you dotty? Have you got bats in your belfry? Are you nuts? Are you daft? Are you psycho? Have you gone ape? Are you out of your gourd? Are you daffy? Are you out of your mind? Are you wacko? Are you mental? Are you a card short of a full deck? Are you a donut short of a dozen? Have you flipped? Are you out of your tree? Are you out to lunch? Are you touched? Are you mental? Are you balmy? Have you become unglued? Have you become unhinged? Are you batty? Are you deranged? Are you cracked? Are you irrational? Are you crazed? Are you cuckoo? Are you not all there? Are you a lunatic? Are you delirious? Are you mad as a March hare? Are you berserk? Are you of unsound mind? Are you off-the-wall? Are you as mad as a hatter? Are you mentally deficient? Have you taken leave of your senses? Did you escape from the looney bin? Are you bananas? Seriously, I wonder why there are so many expressions? Could it be that speaking English make you crazy?

Kind of depends on the context. If you're actually asking whether someone is crazy (mentally unstable) then you could use synonyms for crazy, such as: Are you... loco? deranged? unstable? nuts? off your meds? off your rocker? insane? bonkers? cuckoo? mental? psycho? loony? half-baked? high? demented? certifiable? Have you... gone mad? lost it? lost your mind? lost your marbles? do you... have a screw loose? don't be... absurd. idiotic. ludicrous. a maniac.