twig-instanceof-extension.php

<?php

class LayoutExtension extends \Twig_Extension
{
    /**
     * {@inheritdoc}
     */
    public function getName()
    {
        return 'yolo';
    }

    /**
     * {@inheritdoc}
     */
    public function getTests()
    {
        return array(
            new \Twig_SimpleTest('instanceof', array($this, 'isInstanceOf'))
        );
    }

    public function isInstanceOf($object, $class)
    {
        $reflectionClass = new \ReflectionClass($class);

        return $reflectionClass->isInstance($object);
    }
}

usage.html.twig

{% if object is instanceof('Acme\\Foo\\Entity') %}
    <p>true</p>
{% else %}
    <p>false</p>
{% endif %}