In an application built with Symfony2 we want superadmins to be able to impersonate other users. This is easily done by giving the superadmin user the ROLE_ALLOWED_TO_SWITCH role. The switching is implemented with a call to "somewhere?_switch_user=" as suggesed in the reference documentation.
The problem however, is to detect in a template if the current user is actually impersonated so as to print a link to "somewhere?_switch_user=_exit" on the page, thus enabling the impersonating user to return to her real user.
I haven't been using Symfony2 for a while so I'm not sure, but when you switch to another user you gain all roles assigned to that user and one extra role: ROLE_PREVIOUS_ADMIN. So I guess all you need to do is to use voter to check whether such a role is assigned to the current user using voter.
// Twig
{% if is_granted('ROLE_PREVIOUS_ADMIN') %}
<a href="...?_switch_user=_exit">EXIT</a>
{% endif %}
// PHP
<?php if ($view['security']->isGranted('ROLE_PREVIOUS_ADMIN')): ?>
<a href="...?_switch_user=_exit">EXIT</a>
<?php endif ?>