"Как вы знаете" what does that mean?

Could you please tell me what does "Как вы знаете" means?
Thanks a lot =)

Как вы знаете = as you know

If there is a question mark at the end of the sentence it might mean 'How do you know [about it]'. Even though this use of the word is not right grammatically correct (I think), it is used sometimes:
как вы знаете это? How do you know this?

откуда is a better alternative than как