。
Does she know you?
Literal
She [topic-は] you [object-を] is-knowing [question-か]?
知っている is a famous Japanese 'gotcha' — 知る by itself is a change-of-state verb meaning 'to come to know,' so the present tense 知る would mean 'will come to know.' To express 'knows (currently)' you need the resulting state form 知っている — literally 'is in the state of having come to know.' The negative answer follows the same pattern asymmetrically: '知りません' for 'I don't know,' not '知っていません' (which would imply something like 'I'm in the state of not knowing it yet'). 君 is a slightly old-fashioned, masculine-leaning second-person pronoun — softer than お前 but more direct than あなた.