Here's the code fragment (from /includes/init_includes/init_file_db_names.php) that determines SSL vs. NONSSL based on $_SERVER parameters:
Code:
$request_type = (((isset($_SERVER['HTTPS']) && (strtolower($_SERVER['HTTPS']) == 'on' || $_SERVER['HTTPS'] == '1'))) ||
(isset($_SERVER['HTTP_X_FORWARDED_BY']) && strpos(strtoupper($_SERVER['HTTP_X_FORWARDED_BY']), 'SSL') !== false) ||
(isset($_SERVER['HTTP_X_FORWARDED_HOST']) && (strpos(strtoupper($_SERVER['HTTP_X_FORWARDED_HOST']), 'SSL') !== false || strpos(strtoupper($_SERVER['HTTP_X_FORWARDED_HOST']), str_replace('https://', '', HTTPS_SERVER)) !== false)) ||
(isset($_SERVER['SCRIPT_URI']) && strtolower(substr($_SERVER['SCRIPT_URI'], 0, 6)) == 'https:') ||
(isset($_SERVER['HTTP_X_FORWARDED_SSL']) && ($_SERVER['HTTP_X_FORWARDED_SSL'] == '1' || strtolower($_SERVER['HTTP_X_FORWARDED_SSL']) == 'on')) ||
(isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && (strtolower($_SERVER['HTTP_X_FORWARDED_PROTO']) == 'ssl' || strtolower($_SERVER['HTTP_X_FORWARDED_PROTO']) == 'https')) ||
(isset($_SERVER['HTTP_SSLSESSIONID']) && $_SERVER['HTTP_SSLSESSIONID'] != '') ||
(isset($_SERVER['SERVER_PORT']) && $_SERVER['SERVER_PORT'] == '443')) ? 'SSL' : 'NONSSL';