Check if template exists before rendering

The service holding the twig engine if configured as default is 'templating'.

Inside your Controller do the following:

if ( $this->get('templating')->exists('AcmeDemoBundle:Foo:bar.html.twig') ) {
     // ...
}

The alternative would be catching exception the render() method throws like this:

 try {
      $this->get('templating')->render('AcmeDemoBundle:Foo:bar.html.twig')
  } catch (\Exception $ex) {
     // your conditional code here.
  }

In a normal controller ...

$this->render('...')

is only an alias for ...

$this->container->get('templating')->renderResponse($view, $parameters, $response);

... while ...

$this->get('...') 

... is an alias for

$this->container->get('...')

Have a look at Symfony\FrameworkBundle\Controller\Controller.

The templating service will be removed in future Symfony versions. The future-proof solution based on the twig service is:

if ($this->get('twig')->getLoader()->exists('AcmeDemoBundle:Foo:bar.html.twig')) {
    // ...
}