we can find the symfony version using Kernel.php file but problem is the Location of Kernal Will changes from version to version (Better Do File Search in you Project Directory)
in symfony 3.0 : my_project\vendor\symfony\symfony\src\Symfony\Component\HttpKernel\Kernel.php
Check from Controller/ PHP File
$symfony_version = \Symfony\Component\HttpKernel\Kernel::VERSION;
echo $symfony_version; // this will return version; **o/p:3.0.4-DEV**