Author credentials can be:
Most scholarly - expert with advanced degree (e.g., Ph.D., M.D., J.D.) in relevant subject
Mid-level - journalist, some academic credentials (e.g., B.A., B.S., M.A.) or experience in relevant subject
Least scholarly - no credentials, no academic degree or experience in the subject
For a source you're considering: Where do the author's credentials fall on a scale of 0 to 10?
Look for a brief author biography in these places:
You can also search for information about the author online.
Magazines and Newspapers - magazine and newspaper articles typically provide just an author's name.
You can sometimes learn more on the magazine or newspaper website -- search for the author's name to see if biographical information is provided and also what other articles that person has written. Does the person specialize in a certain area?
Journals - look on the first or last page of the article for information about the author(s), including university and department affiliation.