See what functions are hooked to a WordPress filter.

wp_expose_hooked_functions.php

// Utility filter to see what's attached to a filter:
function dc_wp_show_hooked_filters() {
  // Set the hook name you're checking out here.
  $hook_name = 'pre_get_posts';
  global $wp_filter;
  echo '<pre class="filter-dump">';
  var_dump( $wp_filter[$hook_name] );
  echo '</pre>';
}
add_action( 'wp_head', 'dc_wp_show_hooked_filters' );
add_action( 'admin_head', 'dc_wp_show_hooked_filters' );