Media Bias.

[Tavrobel]

MSNBC and CNN (much more prevalent in the former) are slanted left. Fox News is slanted to the right. Start from there.

However, if you really need to be able to prove the bias, find articles about the same subject, and see what is said about the situation; the differences should become apparent through the analysis of tone and what the author offers to you, the reader as "proof." The easiest way to identify slant is through a polarizing topic, such as abortion (an understatement at best), or the Iraqi War (or the Second Gulf War, for all of those nitpickers).

EDIT: The cheat sheet is as follows:
Left Wing, Liberal, Democrat: Abortion rights are a good thing, "no" on war, and excessive government spending in an effort to create an economic surplus, higher taxes, Big Government, trickle up economy, an unwavering and consistent blame of the Green Party for ruining the 2000 Election for Gore
Right Wing, Conservative, Republican: Abortion is an issue of States' Rights, and therefore, it is bad; "yes" on war, religious fanaticism/fundamentalism, lower taxes, trickle down economy, GOP, Libertarians masquerade as Republicans