well, it's certainly easiest to believe whatever someone tells you than it is to go through life testing everything on your own. the risk in doing that is that you end up believing nonsense just because it's easy. you're right-- you have to draw a line.
i think the line is drawn using use common sense and judgment. for example, i personally believe that russia exists, because there is a vast body of circumstantial evidence that points to it as being true, things such as history, neighbors with russian accents, newspapers, textbooks, media transmissions and so on. so while i have no personal experience with russia, i would say that assuming i can trust other people is a good strategy.
but as for who my parents are? that's pretty iffy. lots of adopted people in this world discover that their parents really aren't biologically related to them at all. so i would say that who my parents are is less reliable than the existence of russia, based on common sense. if someone tells me, i know for a fact that the people claiming to be my parents really are my parents, because they say so, then i tend not to trust his conclusions.
this relates to the flat earth discussion, when someone substitutes faith for facts. too many times people think that just saying lots of people believe something is proof that it is true. if that's the case, then spontaneous generation was true for a long time, and so was the idea that the oceans were full of sea monsters.
i read lots of posts here where the arguments in favor of a round earth consist of nothing more than appeals to authority and appeals to common belief. that's not an argument that has any validity.