How to Determine the Joomla Version
There are several ways to determine the Joomla version. You might need to know this if you were asked to upgrade a Joomla site, fix certain bugs, or to determine if certain plugins/modules are compatible.
1. Determine Joomla version via the Admin Control Panel
The easiest and best way to determine the Joomla version is to log into the admin control panel (for example: http://www.example.com/administrator)
Then select menu “System -> System Information” and there should be the Joomla version…
2. Joomla version in Readme file
The README.txt file at the root of the Joomla install tells you the Joomla version. This file can be accessed via the URL as well (such as http://www.example.com/README.txt) provided that it was not deleted off the server.
For security reasons, some people like to delete this file off the server to avoid revealing Joomla version to hackers who look for outdated Joomla installs. Or better yet, just keep your Joomla up-to-date. Note that the readme file is not totally reliable way to determine Joomla version as the Joomla could have been updated without the readme file being updated.
3. Version of the admin component
You can look in /administrator/components/com_admin/admin.xml
It will show you the version of the admin component, which often is close to the Joomla version.
4. Installation Folder
You might also find the Joomla version in the installation folder — in particular the localise.xml. But typically this folder is deleted off the server.
5. Version clues in the Language Files
You can kind of guess what version Joomla is by the version of its language files. Look in /language/en-GB/en-GB.xml It will have the version of the language file. But the version of the language file is not necessarily the version of the Joomla. But its close.
6. Joomla version in the database
If you have access to and know your way around the database, look for the version_id field in the _schemas table.