Ask services "Which user am I?", as defined by session id?
This service resource simply return an object representing the currently authenticated user:

/**
 * Implements hook_services_resources().
 */
function MYMODULE_services_resources() {
  $resources['MYMODULE'] = array(
    'actions' =&gt; array(
      'whoami' =&gt; array(
        'help' =&gt; 'Get an object referring to the currently authenticated user.',
        'callback' =&gt; '_MYMODULE_whoami_service_resource',
        'access callback' =&gt; 'services_access_menu'
      )
    )
  );

  return $resources;
}


/**
 * Resource callback
 */
function _MYMODULE_whoami_service_resource() {
  return $GLOBALS['user'];
}


Once enabled you should be able to access the resource at http://site.com/[endpoint]/MYMODULE/whoami.json.