User::isExternalUser

static function isExternalUser($userID)
{
    static $users = [];

    if(!isset($users[$userID]))
    {
            if (!ModuleManager::isModuleInstalled('extranet'))
            {
                $users[$userID] = false;
            }
            else
            {
                $dbResult = CUser::getList(
                    'ID',
                    'ASC',
                    array('ID_EQUAL_EXACT' => $userID),
                    array('FIELDS' => array('ID'), 'SELECT' => array('UF_DEPARTMENT'))
                );

                $user = $dbResult->Fetch();
                $users[$userID] = !(is_array($user)
                    && isset($user['UF_DEPARTMENT'])
                    && isset($user['UF_DEPARTMENT'][0])
                    && $user['UF_DEPARTMENT'][0] > 0);
            }
        }

    return $users[$userID];
}