At the root of media bias, is plain old bias, or an inclination towards a position or conclusion, often manifesting in a prejudice. Media bias, then, is simply when media outlets show an inclination towards a particular position or conclusion.
The issue with biased media is that biased news reports present the public with an inaccurate, unbalanced, and/or unfair views of the world around them which will only lead to deeper rooted biases.
You don't need to evaluate bias in the news you consume in a vacuum. There are plenty of sites that fact-check and even rank the bias of various news outlets. Below are links to the two most popular. You can find a quick description of each site's method for ranking media outlets along with information about why you should be wary about the particular sites.
These sites are meant to be a tool to start looking at bias in the media. They should not be the only way you check bias. Even the most thorough methodology will miss nuance when dealing with such a complicated topic. Any time you encounter new information, you need to use your brain to think through what you are reading and not solely rely on a quick graphic.
Reporting news is a human endeavor. It is written by humans, about humans, for humans. Every human being has their own outlook on the world, which will affect how people talk about and view events that happen. This is to say that to be human is to be biased, and those biases will always be reflected in the news to some extent.
There are news outlets that put checks on their own biases, while other outlets embrace their particular biases. While both approaches could be useful, it is important that you know how to recognize a biased source when you encounter one.
Need more help working through the bias of a source? Work through this worksheet:
For more information: Fair.org's How to Detect Bias in Media