Detect where custom post type is declared

Hook into registered_post_type and store the data from debug_backtrace(). The third entry should be the calling plugin.

Sample code

add_action( 'registered_post_type', 'track_post_types', 10, 2);

function track_post_types( $post_type = NULL, $args = array() )
{
    static $tracked = array();

    if ( 'shutdown' === current_filter() )
    {
        print '<pre>$tracked = ' . esc_html( var_export( $tracked, TRUE ) ) . '</pre>';
        return;
    }

    if ( $args->_builtin )
        return;

    $backtrace = debug_backtrace();
    $tracked[ $post_type ] = $backtrace[3];

    add_action( 'shutdown', __FUNCTION__ );
}

Sample output

  array (
    'file' => 'F:\\Dropbox\\wp-content\\plugins\\t5-cpt-tester\\t5-cpt-tester.php',
    'line' => 45,
    'function' => 'register_post_type',
    'args' => 
    array (
      0 => 'project',
      1 => 
      array (
        'can_export' => true,
        'description' => '',
        'exclude_from_search' => false,
        'has_archive' => true,
        'hierarchical' => false,
        'labels' => 
        array (
          'add_new' => 'Add new',
          'add_new_item' => 'Add new project',
          'all_items' => 'All projects',
          'edit_item' => 'Edit project',
          'name' => 'Projects',
          'name_admin_bar' => 'Project',
          'new_item' => 'New project',
          'not_found' => 'No projects found.',
          'not_found_in_trash' => 'No projects in trash.',
          'parent_item_colon' => 'Parent project:',
          'search_items' => 'Search projects',
          'singular_name' => 'Project',
          'view_item' => 'View project',
        ),
        'menu_icon' => NULL,
        'menu_position' => NULL,
        'permalink_epmask' => 1,
        'public' => true,
        'publicly_queryable' => true,
        'query_var' => 'project',
        'rewrite' => 
        array (
          'slug' => 'project',
          'with_front' => false,
        ),
        'show_in_nav_menus' => true,
        'show_in_menu' => true,
        'show_in_admin_bar' => true,
        'show_ui' => true,
        'supports' => 
        array (
          0 => 'author',
          1 => 'comments',
          2 => 'editor',
          3 => 'excerpt',
          4 => 'revisions',
          5 => 'thumbnail',
          6 => 'title',
        ),
      ),
    ),
  )