The credibility of sources is the basis of any accurate research. Basing one’s findings and conclusions on biased, unreliable, and unproven sources significantly reduces the accuracy and credibility of one’s own endeavor. The academic field typically makes use of peer-reviewed sources published in scientific journals, where they face the scrutiny of peers to ensure that no methodological inaccuracies, errors, or biases have made it through. Thus, I think that the first criterion for credibility is whether or not the source is peer-reviewed.
The second criterion, for me, would be the date of publishing. In time-sensitive subjects like contemporary history and certain sciences, time is of the essence. Predictions and conclusions made 10 or 20 years ago may have been disproven by more recent findings. I think a researcher must use the most recent sources, whenever possible. A good threshold would be 5-6 years since the date of publishing.
The third criterion would be the contents of the research itself. Although peer-reviewing and other precautions are in place to prevent bias and errors, one must evaluate how do the findings of one article correlate other results pertaining the research question. Therefore, individual validity based on the researcher’s perspective is important.
Academic sources of good quality can be found in various databases. Although Google Scholar makes a good general search engine, it is better to browse individual databases that have articles sorted out and reviewed for validity. Some of the databases I know are JSTOR, PubMed, ERIC, and ProQuest, among others. Many fields of science have their own dedicated databases to help finding relevant materials easier. This is how I am planning to search for materials for the upcoming research project we are going to have.