When using sources to support your speech points, it matters that you know the perspective of those sources. While no source is is truly "objective," your authority will be enhanced if you are aware of the particular message that your sources are trying to convey.
In order to evaluate a source, ask yourself the following questions:
To help with anaylzing source perspectives, use SourceWatch, "a directory of the people, organizations and issues shaping the public agenda." Or, FactCheckEd.org, an educational resource "designed to help students learn to cut through the fog of misinformation and deception that surrounds the many messages they’re bombarded with every day."