List $wp_admin_bar menu items (debug)

You can use error_log() and print_r():

add_action( 'wp_footer', 'wpse_debug_toolbar' );
function wpse_debug_toolbar() {
    global  $wp_admin_bar;
    error_log( print_r( $wp_admin_bar, true ) );
}

Note that when using print_r() we’ve set the second parameter $return to true so that the results are returned and not echo’d.