It may seem a rather strange question, however, not all foreign learners can understand and interpret the phrase (or part of it) "How do you know?" correctly.
I mean, when we ask "How big is it?" - we mean "to what extent". When "How is it done?" - we mean "in what way". It usually always has a straight interpretation in a straight context. Nevertheless, the phrase "How do you know that? " for instance, can often mean different things even in one context. As far as I understand, however, it should always mean "How come have you obtained knowledge about that?".
If that is so, then does it matter whether it is "Who told you that?", "Where did you find out about that?", "How did you find out about that?" or pure expression of unbelievabilty "Why on earth do you know about that?"