Google announced that they've updated the Robots.txt tester tool within Google Webmaster Tools.
The tool adds three things:
(1) Highlights which line in your robots.txt file is blocking a specific page.
(2) Make test changes to the robots.txt tool and test it before you make the file live.
(3) Google will also show you older versions of your robots.txt file to see past issues.
Here is a test showing you how it highlights the single story I block from search engines on this site:
This a view of the history of my robots.txt file:
Google also issues a subtle hint to try the fetch and render tool again and stop blocking stuff from Google.
Forum discussion at Google+.