Drupal 7
If you want to check exactly what Drupal 7 version you are on, click Reports.
Frrom this screen, either click on Status report or Available updates.
Status report will show you a detailed breakdown of key information about your site, as in the image below which shows your version number at the top of the page.
Available updates will focus even more tightly on the version of Drupal and the module that you're using, as in the image below.
Drupal 6
The Status report and Available updates links also work in Drupal 6, they are just in a very slightly different location. Click on Administer, then Reports and look to the bottom of the list.
