How can you determine a source's biases?

Answer 1

By looking at what they say about a variety of things and see how what they say fits in the spectrum of ideas promoted by various people. Some times just their opinions will make it clear, such as someone who believes Fox News is "Fair and Balanced". In the strongest mode, the source might express opinions that explicitly state bias, "All liberals are traitors." In milder cases it might take looking at how something is written or said or simply what the subjects of interest are: An organization that stressed programs of public welfare would be considered more liberal than one that stressed individual freedoms.