Anthropic shipped a browser-local C2PA checker and is explicit about what it can't tell you.

claude.com/check-content lets you drag in a file and read its content credential without the file leaving the browser, supporting a long list of image, video and audio formats up to 100MB. Two separate mechanisms: text carries a watermark in the writing itself, files get a signed C2PA provenance note attached on download. The concession in their own copy is the useful part, that the tool only detects the credential, so it tells you whether Claude was involved in producing the file and not whether Claude created the content.