Magento

There are two ways to see what version you’re running when you have a running installation.

Login to the admin for Magento:

http://www.yourdomain.com/admin/

...and look in the footer, you should see the following:

Magento ver. X.X.X

Where X.X.X is the version number.

If your admin theme has been modified and you don’t see a version in the footer, go to Magento Connect:

http://www.yourdomain.com/downloader/

...or:

System -> Magento Connect -> Magento Connect Manager

In the Package Name column, look for Mage_All_Latest. The Installed value will also list the version, e.g.:

X.X.X (stable)

Where X.X.X is the version number.

If you only have access to the source code, you can see what version of Magento the source code is based on by opening app/Mage.php and looking for the getVersion() method (should be the first method declared)