If you wish to find out the version of the Joomla application installed on the domain without even knowing the Joomla Admin Panel details ,

You may try to grep  the terms “DEV_LEVE” & “RELEASE”  in the file version.php which is located under the libraries folder of the Joomla installation.

Please use below commands

root@red [/home/abc/public_html/]# grep ‘$DEV_LEVE’ libraries/joomla/version.php
var $DEV_LEVEL = ’22’;

root@red [/home/abc/public_html]# grep ‘$RELEASE’ libraries/joomla/version.php
var $RELEASE = ‘1.5’;