Answer:
- If the source has reliable sources cited (especially if in-text citations are used)
- If the source has a reputation of credibility (ex. Britannica.com vs. Wikipedia)
- If it was written by a professional/professionals (this is what makes Britannica far more credible than Wikipedia- Wikipedia articles are usually written and edited by nonprofessionals)
- If the source is more factual than opinionated (that's where the in-text citations come into play- the in-text citations indicate which information is fact vs. opinion). It's okay if it's opinionated so long as the facts are accurate (an example is a well-cited pursuasive article- it has credible facts to support the author's opinion).
Hope this helps!