One of the worst parts of Magento. Truly hilarious. It checks that a URL returns the string 'MAGENTO' and nothing else as a way to check that it has been installed to the correct place. Impossible to test.

Config.php

<?php

protected function _checkUrl($url, $secure = false)
{
    $prefix = $secure ? 'install/wizard/checkSecureHost/' : 'install/wizard/checkHost/';
    try {
        $client = new Varien_Http_Client($url . 'index.php/' . $prefix);
        $response = $client->request('GET');
        /* @var $responce Zend_Http_Response */
        $body = $response->getBody();
    }
    catch (Exception $e){
        $this->_getInstaller()->getDataModel()
            ->addError(Mage::helper('install')->__('The URL "%s" is not accessible.', $url));
        throw $e;
    }

    if ($body != Mage_Install_Model_Installer::INSTALLER_HOST_RESPONSE) {
        $this->_getInstaller()->getDataModel()
            ->addError(Mage::helper('install')->__('The URL "%s" is invalid.', $url));
        Mage::throwException(Mage::helper('install')->__('Response from server isn\'t valid.'));
    }
    return $this;
}