Do a background check on the author and publisher of the material. Do they support a particular political or religious view that could be affecting their objectivity in the piece? If they are associated with a special-interest group (i.e. the American Library Association or Keep America Safe), this might also be an indication of bias, unless alternative views are presented and addressed with appropriate respect.
Assess the Argument
Identify the author’s main claim. What are they arguing is true or untrue? Pay attention to what the author uses to support their claim – do you find relevant evidence or just emotional examples? Statistics should be used consistently and fairly, with an explanation of where they came from. Check for logical fallacies in the author’s argument and make sure the author considers opposing viewpoints.