Distinguish primary from secondary sources, and evaluate a source's credibility using criteria of authority, currency, and bias.
Research depends on two different kinds of sources, and confusing them undermines a paper's credibility fast. A primary source is firsthand material: the text itself, a historical document, an original letter or speech. A secondary source analyzes or comments on primary material from some distance: literary criticism, a historian's analysis, a documentary. A strong research paper typically uses both -- the primary text as direct evidence, secondary sources to situate that evidence within existing scholarly conversation and to supply context or counterargument your own reading alone could not.
Not every secondary source deserves equal trust, and evaluating credibility is a skill, not a formality. Authority asks who wrote this, and what expertise or institutional backing do they have. Currency asks when this was published, and whether newer research has since revised or replaced its claims. Bias asks what the source's likely angle or interest is, and whether it presents evidence fairly or only what supports one predetermined conclusion. A peer-reviewed academic journal article or a university-press book has typically passed through expert review before publication; an anonymous blog post or an unsourced summary website has not, which does not automatically make it wrong, but does mean its claims need independent verification before you rely on them.
Mastery looks like: They can distinguish primary from secondary sources with accurate examples and can evaluate a real source's credibility using authority, currency, and bias.
Common stumbles: Treating all sources found through a search engine as equally credible; dismissing a source entirely for having a bias rather than verifying its specific claims independently.