Bias is the presentation of evidence in a manner which supports the author's opinion. It does not necessarily make information good or bad, but it is something of which to be aware. In general, one expects an honest person to examine the evidence and then form an objective opinion rather than forming an opinion first and then selecting evidence to prove his or her opinion. However, many people adhere to political, philosophical, or theological positions which color their perception of evidence. This is why it is important to consider the authority of the author.
Some biases are simple to identify. A pharmaceutical company which publishes research on its medications is obviously going to be favorable to its own products because it wants to sell them. In a situation like this you may want to look for research conducted by an outside party. A pro-choice group is going to present a different picture of abortion than will a pro-life group. If you want to examine both sides of the issue, you will want to find information published by both groups, and possibly by a third party as well.
Some biases require familiarity with the author's philosophy in order to identify the bias. In this case, you will want to find out more about the background of the author by reading about him or her, or talking to a professor who knows more about the subject matter.
As you read, remember that no one is an impartial observer. In fact, unbiased information is generally boring because it limits the writer to the indisputable facts. It is the commentary and personal opinions that athors add to the facts which make their writing interesting.