Your site — or your client’s site — has been hacked, and it isn’t functional. How can you tell which version of Joomla the site is running?
Look inside these files — and file location is your first clue:
Joomla! 1.0.x
/includes/version.php
Joomla! 1.5.x
/libraries/joomla/version.php
Joomla! 1.6.x
/libraries/joomla/version.php
Joomla! 2.5.x
/libraries/cms/version.php