How to know the xdebug version i have installed?

php -v command output includes information about installed XDebug version:

$ php -v
PHP 5.6.13-1+deb.sury.org~trusty+3 (cli) 
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2015 Zend Technologies
    with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2015, by Zend Technologies
    with Xdebug v2.3.2, Copyright (c) 2002-2015, by Derick Rethans

or

$ php -v | grep -i "xdebug"

On Debian-based distros such as Ubuntu:

aptitude show php5-xdebug | grep Version

On Redhat-based distros such as CentOS:

yum info php-pecl-xdebug | grep Version

You should be able to do it with a simple test script:

<?php
  phpinfo();
?>

And get output like this: