If you don't have access to the admin the Joomla 2.5 version can be found in the filesystem.
The file is at:
public_html/libraries/cms/version/version.php
For Joomla 2.5 the contents of the file version.php show the release number and version number:
/** @var string Release version. */ public $RELEASE = '2.5'; /** @var string Maintenance version. */
public $DEV_LEVEL = '0';
/** @var string Release version. */ public $RELEASE = '2.5'; /** @var string Maintenance version. */
public $DEV_LEVEL = '0';
