Conspiracy: Fact or Fiction?

We approach the question of conspiracies with a great deal of caution, given that there are so many theological and life-application traps surrounding the matter. How do scientists know that there are, or that there are not, long-term effects from vaccines? With what epistemological authority/confidence could we ever make such an assertion? Nonetheless, there are still some short-term studies available from which families can make some decisions. Then, we answer the question as to whether there are evil people in government, and the extent to which these people exist. We know by their own testimonies as to their evil intent. But how could we know the extent to which they exist? Where do we cross the line into far-fetched surmising on questions like this?