Apply a source-quality rubric -- authority, currency, purpose, accuracy, relevance -- to evaluate potential research sources.
A simple five-point check helps judge any source before trusting it: WHO wrote it (authority -- an expert, a museum, a historian?), WHEN (currency -- is it outdated, especially where understanding has changed?), WHY (purpose -- to inform, or to sell or persuade?), does it match other sources (accuracy), and is it actually about your specific question (relevance). Primary sources come from the time itself, like a medieval manuscript or a merchant's ledger; secondary sources are written later about that time, like a modern historian's book. Both are useful, but knowing which is which matters.
No source is perfect, and even good sources can be incomplete. Part of a researcher's real job is comparing multiple sources and noticing when they disagree -- that disagreement is often exactly where the most interesting research questions hide.
Mastery looks like: They apply a five-point source-quality check to real sources and can explain the difference between primary and secondary sources.
Common stumbles: Trusting a source just because it appeared first in a search; not noticing when a source has an obvious reason to be biased.