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

Well if it's an nonfiction author, of course the information is factual.

Non Fiction - Not Fake. (A trick I always use.)

Now, back to your question - You can tell by seeing if he or she is an expert in that field. Conduct your own research and study on the topic. What sources did the author use? Those should be stated unless it's general information.