Authority isn't enough on its own. Experts often disagree with one another, and in fact, scholarly literature often involves continuing debate. What else makes the source reliable? Let's consider these ideas further:
Is the source peer-reviewed?
Academic articles and books are usually reviewed by other experts before they get published. The reviewers often provide valuable feedback to the authors, which helps make the final product better.
Where did the author get their information?
Does the author provide enough information for you to find their sources (ex. links in a web page, a reference list at the end of an article)?
If the author did an experiment or study, was it well-designed? (You may want to talk to your professor about this.)
Are the author's conclusions logical and reasonable?
Does the author fall victim to any logical fallacies, a.k.a. common flaws in reasoning?
Do you think the author drew the right conclusions from their other sources?
Do you think the author drew the right conclusions from their research?