View source on the page you want to check, copy everything, and paste it here. Nothing leaves your browser — this runs locally, which is why it asks you to paste rather than fetching the URL itself.
What this does and does not check
It checks for the omissions we find most often: missing or invalid JSON-LD, no declared entity, no page type, absent breadcrumbs, question headings without FAQ markup, and the basic metadata that still decides how a result displays. Invalid JSON-LD is worth singling out — it fails silently, so markup can stop working after a release without anything on the page looking different.
It does not validate against the full schema.org vocabulary, check whether your markup matches what is actually rendered, or crawl your site. A clean result here means nothing obvious is missing, not that the markup is correct. That distinction matters, and any tool telling you otherwise is overstating what it can see.