It is always tough to use subjective judgment. Especially when trying to establish the credibility of an author you don’t know.  Here are some treatments of author authority and a quantitative means to evaluate them.

http://mason.gmu.edu/~montecin/web-eval-sites.htm

Vetting secondary source authors and

how the journal of history and computing do it.

So this should give you a guide of where to start in quantitatively assigning authoritative rank.