Instead you add to the stuff you know as things get demonstrated/shown to you to be true. That way you can be reasonably sure that your knowledge is getting closer to the truth over time and that models based on what you know already are more likely to be right.
This is my view of things also. But I do not make any sort of final judgements about things I cannot demonstrate one way or the other. Admittedly I tend to give them a mental grading of how likely they are; but I try not to let that guide my reaction when I find someone who is sure about it. I would rather be wrong because I failed to understand something someone said to me, than be wrong because I rejected it out of hand as being impossible.
I am not saying you have to believe what they are saying is true. What I am saying is that what they believe isn't your responsibility, (nor, as some seem to believe, the right of intelligent people).