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.
One of the ways in which a news source demonstrates its authenticity and responsibility to its readers is through a publicly accessible code of ethics or standards. A sample of various codes, mission statements and ethics handbooks are listed below.