How to know which version of Symfony I have?

Although there are already many good answers I'd like to add an option that hasn't been mentioned. Using the command:

php bin/console about

you can get many details about the current project. The first section is about Symfony itself and looks like this:

-------------------- ------------------------------------------- 
 Symfony                                                         
-------------------- ------------------------------------------- 
 Version              4.2.3                                      
 End of maintenance   07/2019                                    
 End of life          01/2020                                    
-------------------- ------------------------------------------- 

I find the other information besides the version number very useful.

There are also other sections providing details about the (framework) Kernel, PHP, Environment.

Run app/console --version (for Symfony3: bin/console --version), it should give you a pretty good idea. On a random project of mine, the output is:

Symfony version 2.2.0-DEV - app/dev/debug

If you can't access the console, try reading symfony/src/Symfony/Component/HttpKernel/Kernel.php, where the version is hardcoded, for instance:

const VERSION         = '2.2.0';

Just in case you are wondering, console creates an instance of Symfony\Bundle\FrameworkBundle\Console\Application. In this class constructor, it uses Symfony\Component\HttpKernel\Kernel::VERSION to initialize its parent constructor.

Use the following command in your Terminal/Command Prompt:

php bin/console --version

This will give you your Symfony Version.

Another way is to look at the source for Symfony\Component\HttpKernel\Kernel for where const VERSION is defined. Example on GitHub

Locally this would be located in vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php.