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I think that 知るかよ！ means "like I would know!"/"hell, if I know!" and that マジかよ！ means something like "[are you/is this] for real?"/"seriously?", but is there an underlying rule behind the ～かよ pattern seen at the end of sentences?
If I was to, for example, say （それを）食べるかよ, what kind of a meaning would it have?
Can ～かよ also be used as an ordinary question? Does it make a rhetorical question?
か is a sentence final particle that makes a question, and よ is a sentence final particle that adds the subject's attitude. Ignoring the meaning added by よ, which does not affect the core meaning of the sentence, all of the sentences you have are questions. And in all of your examples, they are rhetorical questions.
知るかよ
literally: 'Do I know/bother about it?'
'I don't know/bother about it!'まじかよ
literally: 'Is that serious?'
'I can't believe it!'食べるかよ
literally: 'Will I eat it?'
'I won't eat it!'
Note that the addition of よ adds subject's attitude, which makes it difficult to interpret the sentences as literal questions.
知るか。 / 食べる（もん）か！ are both easily recognizable as non-questions even without よ. However, 食べるのか seems (to me) to be a literal question without よ.
Apr 7, 2012 at 13:27
食べるのか can be rhetorical in several ways: "Are you ever going to eat it?", "Am I supposed to eat it?", etc.