New Google Webmaster Tools
The new - and long overdue - Google Webmaster Tools console features the one report we really wanted but didn't have: the index status of pages on a site. Previously it's been possible to piece together a reasonable assessment of this data by a process that essentially amounted to divination but now we have access to data.
The new index status report shows pages in a number of different, and well documented categories.
For illustrative purposes this report is showing all the things but there are a lot of filters and drill downs which allow you to get into the details at the granularity of individual URLs - even on big sites.
By default the report shows all pages Google has found but it's also possible to filter at the top level by sitemaps which makes them a significantly more useful tool. If you have all the pages you care about in the sitemap then it isolates the signal from the noise - you can quickly drill down and see exactly which pages need attention. Briding the gap there is a report on the full report that shows pages that are indexed but not in the sitemap which can be used to help you identify what to add to your sitemap.
On sites I've been working closely with the index status report hasn't shown up anything that surprised me but it's already making it much easier to assign priorities and track progress of SEO work.
If you'd like some more help with understanding how your site's being indexed then please get in touch.