WordPress has several functions call pluggable functions essentially, they are created to be overrided for brand new functionality, but there's a catch - if another wordpress plugin define that function first, another wordpress plugin of the lower priority cannot...(otherwise you will see a fatal error). Hence it's quite common to possess 'include guards' such as this

if (!function_exists('wp_new_user_notification'))
{ 
   function wp_new_user_notification($userid, $blah='')
   {
   }
}

This eliminates fatal errors, however it results in plug ins inexplicably failing...

Usually one avoid contrasting plug ins. One problem, though, is you have no clue if your wordpress plugin clashes, and often it's difficult to prevent that for feature-packed plug ins.

Now you ask , (sorry to take such a long time to create the premise) how can you identify clashes? It is usually a hidden failure since the function is available, no error is tossed, however the result's "Hey, this plugin's damaged!"

I possibly could echo something if your function continues to be defined, but how do you transform it into a unit test? WordPress would come with all plugins' header files each time a page is load. How do you determine the pluggable function which Wordpress has loaded may be the one I would like, programatically?

The only real factor that involves thoughts are to mark the function declaration failure inside your application and test for that marker:

if (!function_exists('wp_new_user_notification'))
{ 
   function wp_new_user_notification($userid, $blah='')
   {
   }
}
else
{
  define('MY_PLUGIN_NEW_USER_NOTIFICATION_DECLARATION_FAILED', true);
}

after which test with defined:

if(defined('MY_PLUGIN_NEW_USER_NOTIFICATION_DECLARATION_FAILED'))
{
 // fail
}