You know as a parenthetical filler is from 1712, but it has roots in 14c. You know as a euphemism for a thing or situation unmentionable is from 1867; you-know-who for a person it is thought best not to name (but implying the hearer knows) is from 1840.
As an expression of surprise, what do you know attested by 1914. Don't I know it in the opposite sense ("you need not tell me") is from 1874. You never know as a response to something unexpected is attested from 1924.