Magento How to debug blank white screen

This is how I got it corrected(Hope will help you guys):

1. Use the following code in your index.php file

   ini_set('error_reporting', E_ERROR);
   register_shutdown_function("fatal_handler");
   function fatal_handler() {
       $error = error_get_last();
       echo("<pre>");
       print_r($error);
   }
   

2. In my case it tolde me that error/503.php was unavailable.

3.The issue was with testimonial extension I used(http://www.magentocommerce.com/magento-connect/magebuzz-free-testimonial.html)

4. I deleted the testimonial.xml file in my app/etc/modules/testimoanial.xml.
5. delete “maintenance.flag” file.