Asking someone "How did you know this?" implies that whatever they knew was the truth. Is there some other straight-foward answer that only asks how did the person come up to know this without actually implying that it is the truth?
2 Answers
If you want to talk about what a person believes to be true, without implying that it is true, you could use:
Why do you believe that?
But this has the opposite effect: it suggests that you think that what they believe is not true.
One might say "Where did you get this information" or "How did you manage to acquire this information". There are many ways to ask this, just change a few words to see which suits you best to "not imply that it is the truth"