When should you question a sources credibility

You should question a source's credibility prior to using it.

One should question: if the source is current (what is the date of the research), who is the author of the source (what authority/knowledge/experience do they have), what depth does the source reach, who is the audience, what is the purpose of the source, etc.

If you want to be sure you get accurate information then its important that you use credible and unbias sources.