If you find a bug with your Sitecore website, one of the first things Sitecore support will ask you for is the Sitecore version number that the site is running on. If you forget, then finding out which version a site uses is pretty simple.

Go to the Sitecore Desktop

In your web browser go to:

sitecore_getting_version_number

In the 'Start Menu', select 'All Applications' -> 'System' -> 'About Sitecore'

sitecore_getting_version_number_1

At the top, you can see the Sitecore version and revision number. As long as you have access to the Sitecore desktop, you can find out the Sitecore version within a few seconds.