How can you tell if an author of a nonfiction piece is credible?

The answer is: if the author is considered an expert in his or her field

In order to know the credibility of the author, you just need to conduct a quick  research regarding the author's educational background and his/her working experiences. IF the educational background and the working field match the topics on the non-fiction books, that we assume that the author has a credibility.

answer; emotional intelligence (ei);