Last month, Google introduced Site Diagnostics feature to AdSense.
AdSense publishers will notice a new tab when they log into their accounts next – ‘Site Diagnostics’.
The feature was not longlived, though.
Within a few hours of my post on it the ’site diagnostics’ tab disappeared from AdSense publisher’s reports pages and it has not been seen since.

The feature is now back. AdSense publishers can see the tab “Site Diagnostics” next to the “Report Manager” when they log into their accounts. This tool displays diagnostic information about the pages Google crawls to display relevant ads. So if AdSense ads don’t display, you can check the problem here.