If you just hear the sentence and can't tell if it is spelled with a capital or lower case 's', then there is no way to tell without the context. So if you just heard someone say this sentence by itself, you would have no way of knowing which meaning it has.if you see it with a capital 'S', then it would mean 'you,' but with a lower case 's', it could mean either her or them. It could actually mean 'it', if it is referring to a feminine noun (e.g.- "Ich kenne die Stadt." "Ich auch. Ich kenne sie.")
It can be the first word in the sentence if it is a question (Kenne ich sie?=Do I know her?), or with the German case system the object can go in the first place in the sentence: Sie kenne ich=Ich kenne sie, but putting 'sie' first puts emphasis on the fact that 'she' is the person that you know. The closest English equivalent would be to say "It is her that I know" or just to put verbal emphasis in the word 'her' when saying, "I know her."
Kennen is used when one wants to say they know a person or a place or particular object. " Ich kenne sie." "Du kennst Berlin." Wissen is used to express one knows a fact or knowledge. "Er weiß Englisch." "Ich weiß seine Muttersprache."
I'm going to add to this to be a little more clear: Kennen would be used in reference to a noun or pronoun and wissen is for what is going on with things (usually expressed with verbs or subordinating clauses). e.g., "Ich kenne das Bier." vs. "Ich weiß, wie Bier schmeckt." (I know that beer. vs. I know how beer tastes.)
How does "Kennen" declined for the rest of pronouns?? Like "Ich kenne" " Du kenst" and....