The "Structured Data Dashboard" in Webmaster Tools introduced today, provide webmasters with greater visibility into the structured data that Google knows about for their website.
"Webmasters can use the Structured Data Dashboard to verify that Google is picking up new markup, as well as to detect problems with existing markup, for example monitor potential changes in instance counts during site redesigns," Google stated.
Google makes use of structured data in a number of ways including rich snippets which allow websites to highlight specific types of content in search results. Websites participate by marking up their content using industry-standard formats and schemas.
The Structured Data Dashboard has three views: site, item type and page-level as explained below:
At the top level, the Structured Data Dashboard, which is under Optimization, aggregates this data (by root item type and vocabulary schema). "Root item type means an item that is not an attribute of another on the same page. For example, the site below has about 2 million Schema.Org annotations for Books ("http://schema.org/Book")," explains Google.
Itemtype-level view provides per-page details for each item type, as seen below. The default sort order is such that it would facilitate inspection of the most recently added Structured Data.
Google parses and stores a fixed number of pages for each site and item type. They are stored in decreasing order by the time in which they were crawled. "We also keep all their structured data markup. For certain item types we also provide specialized preview columns as seen in this example below (e.g. "Name" is specific to schema.org Product)," explained Thomas Biggs & Andrei Pascovici, Webmaster Tools Team.
Page-level view showing all attributes of every item type on the given page (as well as a link to the Rich Snippet testing tool for the page in question).