Site Indexation Tool

It is of the most importance to ensure that all the key pages of your website
have been well indexed by the major Search Engines.Google help the addition
 of indexation data within Webmaster Tools. To see the newly revealed indexation
information, simply log in to your Google Webmaster Tools.

The  data get that:
    How many pages are blocked by robots.txt
    How many pages it has not to include in the index
    How many pages  has crawled

See Webmaster Tools messages frequently, you could miss
an important piece of information.