Green’s Dictionary of Slang

you know it (is) phr.

(orig. US) any form of emphatic agreement, yes indeed, you’re right etc.

[US]W. Diehl Sharky’s Machine 224: ‘And Domino is independent?’ ‘You know it. A no-shit lady.’.
[US]N. George ‘Pop Life’ in Buppies, B-Boys, Baps and Bohos (1994) 99: Are they qualified? Hell, no. Do their sons crave this newfound attention? You know it.
[US]Eble Sl. and Sociability 101: [...] that bites, that bites the big one, you know it, and you’re hating it are various ways of expressing commiseration in the face of misfortune.