Fact-checking, i.e. internal control. Look for editorial team, content standards, and ethics statements.
Credentials: is the author an expert in the topic? Hardly infallible, but generally a good sign.
Reputation in known reliable sources (network of trust). Presumably a reliable source does not cite unreliable sources (excepting quotes and such).
Interests, e.g. commercial or advocacy, in distorting the truth. Applies to companies, non-profits, and governments alike. May be usable with attribution ("according to...") if not in Wikipedia voice.
Check for copying material e.g. from press releases (bad)
Distinction often not absolutely clear. Classification depends on the statement being made in addition to source itself (e.g. facts stated by book as secondary vs. information about the book as primary).
Think in terms of process between levels: primary is original reporting of topic. Secondary interprets facts from primary. Tertiary summarizes/synthesizes/compiles secondary (and primary).
"News media" (newspapers, etc.) can be primary and not secondary. Look for analysis, evaluation, interpretation, or synthesis of the facts (WP:NOR). Quoting an eyewitness, reporting on announcement, etc. by itself is primary.
Research article ("we conducted a study") is primary. Review article (literature review, systematic review, etc.) is secondary with respect to the original research.
