Another thing to note, in addition to the excellent writeup above, is that bias in news reporting is mostly limited to how things are said or what isn't said. Mainstream western journalism is very unlikely to actually lie, however there are many other methods journalists can use to promote their viewpoints. A good experiment to do is find a polarizing topic and read two articles from sources with opposite viewpoints. Instead of falling into the common trap of looking for factual inaccuracies or logical fallacies, take note of what each side mentions that the other side doesn't. In addition to this, look at any pictures that might accompany the article (for instance, an anti war article might include a picture of civilian casualties while a pro war article might have a picture of aircraft flying over enemy fighters), and look at any interviews conducted, focusing not only on who was interviewed but also how rigorous the questioning was.

   One way that news sources can slip outright lies into their coverage is by interviewing someone who believes in a discredited claim and failing to correct them. For instance, according to a report by Fox News interviewed 52 people who claimed that Barack Obama was not a US citizen but only corrected 8 of these claims. Since Fox is a mainstream news outlet, people listening to their coverage may assume that these unchallenged claims are true, since reporters are ethically bound to report the truth. However biased reporters can give the impression that false claims are true by using interviews to give air time to believers of these claims

    An incomplete list of items to check for bias would include: headlines, music, images, video clips, captions, amount of time/space dedicated to different angles, people interviewed and how they are treated, buzzwords (such as extremist, radical or fundamentalist), graphics (including colors used), sources  cited, the source (reporter and distributor), placement of the story, word choice and any questionable statistics. 

Although my politics are generally liberal, I can consistently find both liberal and conservative biases in most news sources. The example of Fox News was not used to bash Fox but simply because it is probably the most obvious and flagrant bias I have ever seen