I haven't understood what even means here:
How do you even know that?
and what's the difference with the following?
How do you know that?
I haven't understood what even means here:
How do you even know that?
and what's the difference with the following?
How do you know that?
the word "even" is used in this to show somewhat an act of surprise.
e.g "I can't believe your amount of knowledge, how'd you even know that?"
The difference between the two statements are the use of expressions. The word "even" provides a more stressed toned rather than simply questioning their comprehension.
I think it is a term of wonderment that I can't believe you knew that rather than a simple question asking for a source
This usage is picked up on in the Urban Dictionary
even word added to a sentence when you want to stress something hysterically just to sound angrier.
'Why are you EVEN trying it?'
'What are you EVEN doing here?'
Though both these examples strongly suggest other verbs with implications even more laughable (as required by stricter definitions in other dictionaries), the loose definition given here works with say
'How do you even know that?'
where one might well ask 'Know it as opposed to . . . ?