there is a column which shows which requests have urls that were rejected. You have to go back pretty far to find requests with rejected URLs (since there are a lot of requests and urls are rejected only rarely).
A third party is free to go through googles transparancy reports, there take down requests, and use the engine itself to validate what got taken down. As far as I am aware, Google is the only party devoting the resources nessasary to go through their DMCA requests.