Bias

According to Merriam-Webster's Collegiate Dictionary, bias is "an inclination of temperament or outlook; especially: a personal and sometimes unreasoned judgment: PREJUDICE." All authors have inclinations of outlook and are to some degree biased, but bias damages the reliability of some works more than others.

Signs suggesting bias

the work is created by a person or organization that is committed to a viewpoint that may color their views on the subject at hand
the work is published by a person or organization that would benefit by promoting a particular point of view
the work is actively trying to sell a product or service
the work does not document information or cites unscholarly or biased works

The Cautious Researcher Should

learn about the author of a work to identify particular interests and commitments that may lead to significant bias
independently verify facts and statistics with a reliable source
consider whether the biased analysis is the only one that fits the data
try to identify general and specific ways in which the bias may have influenced the argument

For example: Consider how the author's point of view affects the argument.

The author claims that Buddhism and Mormonism are cults. How would his religious and academic background influence your opinion of his arguments?