Peer-review (see definition) is one major way of evaluating the credibility of a scholarly published source. Many databases (including Communication and Mass Media Complete) offer search filters for peer-reviewed articles, as in this example:
To identify scholarly journals, they have already consulted master lists such as Ulrich's International Periodicals Directory. However, certain types of articles (such as News, Notes, or Book Reviews) in a scholarly journal may not go through the peer-review process.
Impact factor is another way to evaluate the attention authors have received from their scholarly peers. It refers to the number of later articles citing (putting a citation to it in the text and/or biblography) a particular article or journal. You need not look up the impact factor of a given article before you evaluate it for its utility to your research proposal. However, watch in your search results for "Times cited in this database" when you are in Communication & Mass Media Complete.
Clicking that will lead you to other relevant articles that have mentioned the original citation you were examining. This is a good indicator of impact factor for that article, and it helps you continue your search forward for a particular topic.
Authority of the source is one factor among several to evaluate before you use it in your research. Consider the CRAAP test from Western University for other criteria. If you're unsure about whether to employ information published from a website, use this checklist for evaluating websites from University of Maryland Libraries.