Lucent
New science explained in plain language, retraction checked.
/check is a free tool from Lucent: paste a DOI and it reads Crossref's public metadata for retraction related fields (update-to, updated-by, and retraction relations). It states plainly when it finds a retraction notice, when it finds none, and when Crossref has no record at all, since an unresolved lookup is not the same as a clean paper. No account, no API key, no cost.