Check if template exists before rendering

Maybe also an option:

{% include 'AcmeDemoBundle:Foo:bar.html.twig' ignore missing %}

The ignore missing addition tells twig to simply do nothing, when the template is not found.

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')) {
    // ...
}

In case you need to check for template existance from inside twig templates you have to use the array include methods, as described in the documentation:

{% include ['page_detailed.html', 'page.html'] %}