Fact- many people only read the first few paragraphs of a story, and skip the rest.
So in a news story where there are 2 different sides, if the story begins on paragraph 2 with quotes exclusively from one side, then doesn't begin quoting the other side until the middle of the story, then you can be sure many readers only read the quotes from the side covered first.
A balanced story should have views from both sides fit within the beginning of the story. Just something to notice as you read the news.
This is not necessarily a good indicator of bias; Lunar is right that many stories are partisan just by the nature of the subject they are covering. Plus, news articles don't necessarily need to have a spin in both directions, so long as they're factual. Several recent New York Times articles on the investigation into the CIA leak case may seem biased against the administration, but in reality they're just presenting the honest and most important facts -- they just don't look good for the administration.
News organizations should never be held to trying to represent everyone's point of view -- they should report facts alone. Article leads should always contain the most important information of the story, and if that excludes one side, so be it.
I didn't do much reporting in my brief foray into the world of journalism, but here are a couple of examples of a murder trial that I covered. In the first, I remained as dry and factual as possible:
http://www.kstatecollegian.com/article.php?a=5180In this next one, a few weeks later, I focused more on the family of the victim, because they were the big story at the time.
http://www.kstatecollegian.com/article.php?a=5466 The articles had a different feel because the story had changed. I think the same goes for any news article -- the article leads should never have to be "A Republican says this, a Democrat says this" just because the general public is too impatient to read a full story.