See the definition of bias from the Merriam-Webster Dictionary online: Bias
Explore different kinds of cognitive biases: List of Cognitive Biases
What is the purpose of the source? (Does it inform, persuade, present opinions, report research, or sell a product?
Who is the publisher of the source? (Is it an organization, association, or company?)
Who is the intended audience of the source? (Is it for the general population or for a select group of people?)
Where is it published? (Is it local, regional, national, or international?)
Who is the author? (Is an author identified? Are any credentials listed? Is the author qualified to speak on this topic? What other topics has the author written about? Does the author belong to a partisan organization that may influence point of view?)
What is the purpose of the article? (Is it a review, commentary, editorial, informative article, research article, etc.?)
What kind of supporting material is offered? (Does the author quote research studies, statistics, or use personal anecdotes or experiences? Is logic applied when drawing conclusions? Are deceptive arguments used?)
What kind of language is used in the article? (Are inflammatory, loaded, or emotionally-charged words used?)
24 Cognitive Biases That Are Warping your Perception of Reality
21st Century Information Fluency: Indicators of Bias
Citizen Source: Check out the listings of political journals and Think Tanks and Policy Institutes
AllSides: Follow the "News" or "Issues" sections to see how individual news stories and sources are rated on a range or liberal, center, or conservative.