I recently came across a post with the title "Is the New York Times Biased?" My immediate reaction was to ask not what the answer was but what the question meant. There are a lot of stories out there and no newspaper can cover all of them, so how do we judge the selection of what to cover?
One basis for deciding what to cover, common to practically all news sources, is what you think your readers will find interesting, but that was probably not what the author of the post was thinking of. Another is what you see as important and informative. That will, inevitably, depend on your view of the world. If you believe that a lot of policemen are irresponsibly violent, go around smashing down doors, shooting dogs with no good reason and beating unarmed victims to death, you will see an example of such behavior as important—this is a big problem people need to know about—and informative, since it teaches a lesson about the world that you think is true. If you believe that policemen are generally responsible and restrained in their use of force, you may see the same incident as experimental error rather than data, an exception due to a single bad apple—assuming you believe it at all. Probably not worth covering.
Suppose you do cover the incident. You are likely to look for, believe, and report evidence that fits your prior views, be skeptical of evidence that does not. If you are sufficiently honest to report the latter, you will do so only after going to a good deal of trouble to make sure it is true, more trouble than you go to with regard to evidence that supports your beliefs. The result will be a pattern of coverage that tends to support what you already believe.
In order to conclude that the New York Times' selection of stories to cover is biased, I need to compare it with how many stories on each side are out there to be covered and how important and informative they are. My view of that will reflect my own view of the world. In my case, not only does the selection of stories by the New York Times strike me as obviously biased against free markets, so does the selection of stories by the Wall Street Journal. The Journal is more favorable to the market than the Times but not nearly as much more as I am. I conclude that to describe a news source as biased says little more than that its view of the world is substantially different from mine.
There are two other criteria for judging news sources that are, in my view, both more objective and more useful: honesty and competence. For an example of the first, consider the Huffington Post. Some years back when I was following news stories about how nutty various Tea Party candidates were said to be, one of them dealt with a candidate claimed to be opposed to the separation of church and state. I found a story on him on the Post website. It included a video of the talk the claim was based on from which it was clear that he supported the separation but disagreed with some interpretations of what it implied, and the story was consistent with that. I concluded that while the Post might have a strong left wing bias, it was also honest.
For an example of the other criterion, consider a story I read years ago in the Wall Street Journal dealing with the adoption market. It was described as a situation where the free market did not work, since there was a shortage of babies to be adopted. The article never mentioned that this was a market with price control at a price of zero, it being illegal to pay a mother for permission to adopt her child. It is possible that that occurred to the authors and they decided not to mention it, in which case the article was dishonest. But I think it more likely that, because the authors were not accustomed to thinking like economists, the role of price in equalizing supply to demand simply never occurred to them. In which case the article was incompetent but not dishonest.
To control for bias, get your information from a range of sources. If you want it to be reliable information, try to find sources that are, so far as you can tell, both honest and competent.