On the site you can see which individual requests were rejected (or partially rejected).

e.g. http://www.google.com/transparencyreport/removals/copyright/...

if you look through the page of all requests

http://www.google.com/transparencyreport/removals/copyright/...

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).

e.g. http://www.google.com/transparencyreport/removals/copyright/...

I applaud Google for at least trying to do the right thing, but how do we know what percentage of false positives/negatives are in there?

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.