One awesome and dangerous thing about learning Japanese is that uber-casual, often rude phrases tend to be really simple.

Today's bundle of rudeness:

知るかよ！
shiru ka yo!
How (the hell) should I know?

Yeah, if you want to make a rude, rhetorical question, all you need to do is snap ～かよ (~ka yo) onto the end of it.

You're most likely to see it as one of the following:

[(Dict. Form) Verb] + かよ
[Noun] + かよ
[Adjective] + かよ

Really, though, it can just latch out to any sentence that is not a request or command. This makes sense in English, too. You can't really combine something like "come here" and "are you serious?" into one clause... I think... right?

Since this is pretty rude (although still quite common), we can pull lots of examples from anime...

いいじゃねぇかよ
ii ja nee ka yo
Come on, it's cool.
(Literally: "good + is not + かよ")
(Note: ～ない [~nai] becoming → ～ねぇ [~nee] is also pretty rude/casual.)

分かんねぇこと言ってる場合かよ
wakannee koto itteru baai ka yo
Is this really the time to be blabbering nonsense?!
(Literally: "don't understand + thing + saying + situation + かよ")
(Note: Here we have 分からない [wakaranai] becoming → 分からねぇ [~wakaranee] becoming → 分かんねぇ [wakannee]. So much casual. *_*.)

アイツが成績トップってことかよ
aitsu ga seiseki toppu tte koto ka yo
So that means he's the top of the class!
(Literally: "he [rude] + が + grades + top + ってこと + かよ")

(Note: ～ ってこと　[ ~ tte koto ] is a casual abbreviation of ～ということ　[ ~ to iu koto ], which the closest grammar dictionary I have on hand defines as "[conclusion] in other words." Which is to say "that means," which we see in the translation above. Realistically, this point needs it's own [several] lessons, though.)

Pronunciation Nerds, Beware

Notice that the intonation drops on the よ (yo). In other words, don't use a rising intonation (i.e. don't pronounce this rhetorical question like a genuine question).

How to use this grammar:

If you're at an advanced level, just snap ～かよ onto nouns and the plain forms of verbs and adjectives. If you're just getting started, I'd only attach it to the dictionary form of verbs:

A: Hey, where'd your money go?
B: 知るかよ！
shiru ka yo!
How should I know!