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this sentence is from "how I met your mother"
- You know how I've always had a thing for half-Asian girls?
And it got me thinking about the differences between the followings
- You know that I've always had a thing for half-Asian girls?
what's the difference between them?
Technically, how means "in what manner" or "in what way". That just means "the thing specified". So
"You know how I like anchovies on pizza?"
means "Are you aware of the degree or manner in which I like anchovies on pizza?" while
"You know that I like anchovies on pizza?"
means "Are you aware of the fact that I like anchovies on pizza?"
But idiomatically, "You know how..." is used to gently remind someone or ask for confirmation; it's the kind of thing you would say when you are fairly certain that the person already knows the fact, but you want to establish its importance to the next thing you are about to say. In your example, the speaker is probably already certain that the listener knows that he has a thing for half-Asian girls, but he is reminding the listener because of its important for the next statement, that he has a new favorite.
Here's an English.SE discussion of "how vs. that": “How” vs. “that” in “You know how we have pizza on Thursdays, right?”
, right? to the expression makes it somewhat interchangeable with the first example.