How are knowledge claims justified?
There are may ways in which to answer this very good question. The sociology of knowledge is an important field and the basic idea behind this could be summarized in three points.
First, all knowledge is socially constructed. This simply means that knowledge claims are justified when a society sanctions them. In other words, what is true and untrue is based on what society in which you happen to live. So, what is true in one place in the world is different than in another, because knowledge claims are different based on different cultures.
Second, those in power are the ones who create society. So, in view of this, those in power are usually the ones that justify true claims. Theorists like, Berger, Foucault, and Bourdieu all make this point. Here is a quote:
"Each society has its régime of truth, its ‘general politics’ of truth: that is, the types of discourses which it accepts and makes function as truth; the mechanisms and instances which enable one to distinguish true and false statements, the means by which each is sanctioned; the techniques and procedures accorded value in the acquisition of truth; the status of those who are charged with saying what counts as true."
Third, what is deemed truth and not true is based on what approximates the justified knowledge claims of a society. We call these plausibility structures. Peter Berger has done good work in this area. These points should get you started.