If you are given the English first, ie "are you crazy", you can translate as follows:
tu es fou (1 male, familiar) tu es folle (1 female, familiar) vous êtes fou (1 male, polite) vous êtes folle (1 female, polite) vous êtes fous (males or males+females) vous êtes folles (females)
Nice, isn't it?
Sitesurf, I got this question in the "select he correct answer" exercise. My choices were fou, folle, fous all of which could work, as you indicated, depending on whether you are addressing a single male, a single female or many people including at least one male. So I don't think any of those should be marked wrong for that exercise. Or perhaps you can just get rid of it altogether.
With regard to whether you can vouvoyer someone and ask them if they are crazy, I say, "Absolutely!" If you had a boss who was a pervert, and he said something untoward to you, surely you can blurt out, "Are you crazy?" to him and if you still want to grant him respect, as Northernguy pointed out, or you could decide he is a jerk and not worthy of such respect and tutoyer him.
3 years ago... already!
Yes, "fou, folle, fous and folles" would all work with "vous". I therefore corrected the exercise.
And yes, you can definitely say this to a single male (of course there are other versions, with "tu" and a choice of other adjectives) - I surely do, when some crazy driver behaves badly!