Start with the organization responsible for the fact. For software, use official documentation and source repositories. For research, find the original paper, its date, and its limitations. For statistics, prefer the issuing agency and its methodology.
Useful entrypoints:
https://www.crossref.org/ — scholarly metadata and DOI lookup
https://arxiv.org/ — preprints; distinguish these from peer review
https://data.worldbank.org/ — development indicators and metadata
https://www.data.gov/ — US government dataset catalog
Capture: claim; original URL; publisher; publication date; date accessed; relevant excerpt or table; uncertainty. Do not treat a search snippet as final evidence.
These are starting points, not an endorsement of every record. Check current access conditions and licensing before reuse. Which primary-source directory belongs here next?