Google introduced "Manual Webspam Action" viewer feature in Webmaster Tools, a much awaited feature requested by webmasters and website owners. Google have been taking manual actions for the violation of their webmaster quality guidelines, and due to that many faced downfall in keyword rankings, few received the messages confirming the same and few witnessed lower ranking without getting any messages in WMT. So many webmasters wanted to see what's gone wrong with their website OR is it due the fact that Google have taken any action against their website. With the introduction of "Manual Webspam Action" button, anyone can see if the action is taken or not.
This feature is located under "Search Traffic" Section named as "Manual Actions". Clicking on this will display any action taken with details; otherwise it will read as "No manual webspam actions found."
Google also confirmed that only less than 2% are such cases which are removed/demoted from Google with manual action, so the majority of sites will see as "No Action Found". If your site has been found having manual action, probably you will see detailed information mention like if it is "Site Wide" Or "Partial Match" and type of problems like "Unnatural Links", "Hidden text" or "Pure Web Spam" etc (Read here in detail about all included web spam types).
In case of "Partial Match", It will also show you the list of URL's which have been taken action against. There is also a "Request a Review" button for such sites to request a quick reviews once you have fixed the problem.