Will MacAskill has been posting several articles on the EA Forum. This one seems especially interesting for a LessWrong audience. In it Will approaches the title question by attempting to create an outside view prior, and then updates based on our inside view evidence. Great discussion follows in the comments.
This is a linkpost for https://forum.effectivealtruism.org/posts/XXLf6FmWujkxna3E6/are-we-living-at-the-most-influential-time-in-history-1