Even typically reliable sources, whether mainstream or alternative, corporate or nonprofit, rely on particular media frames to report stories and select stories based on different notions of newsworthiness. The best thing to do in our contemporary media environment is to read/watch/listen widely and often and to be critical of the sources we share and engage with on social media.
Here are some websites that can help you identify media bias:
CounterSpin: Weekly radio show that critically examines major news stories and that addresses issues that mainstream media may not have addressed. Seeks to explore biased and inaccurate news and censored stories.
On the Media: Weekly radio show on how the media shapes our world view (from WNYC)
ProPublica: An independent, non-profit investigative journalism newsroom. Seeks to exposee exploitation and to serve the public interest.