A new February 2019 paper, from the German Centre for Higher Education Research and Science Studies, testing existing methods for auto-detection of OA papers in Web of Science (WOS) and Scopus. The conclusions are about what you might expect — that it’s easier said than done, even with such well-behaved services, and even then it’s partial.

But as part of the study a research assistant valiantly undertook further manual checking by hand. They found that OA full-text links there were broken at a rate of 17%…

“a further manual check found about 17% of OA publications are not accessible … 17.57% in WOS and 16.74% in Scopus.”