The first thing to do is to add the following code to your wp-config.php file. This file is located at the root of your WordPress install.
if ( isset($_GET['debug']) && $_GET['debug'] == 'debug')
define('WP_DEBUG', true);
Once done, simply add a GET parameter to the url of the page you’d like to debug, as shown below:
http://www.wprecipes.com/contact?debug=debug
Thanks to Joost de Valk for this great tip!
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How to easily enable/disable debug mode in WordPress