How do I check whether a legal open copy of a paywalled paper exists?

Query the paper's DOI against an open-access location service such as Unpaywall, then check repository aggregators and the publisher page for an accepted manuscript; if none of these surfaces a lawful copy, record that only the paywalled version was found. Unpaywall records describe open-access locations associated with a DOI, including host type, version, license information when available, and a selected best open-access location, so the DOI is the most direct starting point.

1. Run the DOI through Unpaywall. Enter the DOI in the Unpaywall lookup and read the returned record. A listed open-access location gives you a host and version to open; the record's selected best location is the first place to try. If the record lists no open-access location, move to the next step rather than concluding the paper is closed.
2. Check repository aggregators. Search CORE, which aggregates open-access research outputs and offers discovery services and tools for finding repository copies of scholarly work. A repository copy may appear here even when the DOI lookup returned nothing, because repositories host accepted manuscripts as well as published versions.
3. Check the publisher page itself. Look for an accepted manuscript or author version hosted on the publisher's site. Publishers sometimes post an accepted manuscript alongside the paywalled version of record, and the Unpaywall record's version field tells you which version a listed location holds.
4. Check open-access journal records when relevant. If the article appears in an open-access journal, search DOAJ, which provides searchable article and journal records for open-access journals meeting its inclusion criteria. A DOAJ record confirms the journal's open-access status; it does not by itself confirm that this specific article is freely available, so verify the article page.
5. Document the outcome. If steps 1–4 produce no lawful copy, note the DOI, the sources checked, and the finding that only the paywalled version exists. That record lets you decide whether to request the paper through your library or contact the authors.

A repository hit and a publisher-page hit can point to different versions; check the version field before citing, since an accepted manuscript may differ from the published version of record.