Simon Prosser

Debugging tool for theme authors and reviewers.
This tool intercepts all debug information and prints it all out neatly into the footer. It also checks each page for W3C validation. 
This plugin is released as a tool to aid the development of themes and plugins for WordPress and can be used to aid debugging your theme before submission to the themes directory.

Debogger

Debugging tool for theme authors and reviewers.

Jake Spurlock

This plugin will show you what post meta is attached to the main post.

Debug Bar Post Meta

I am not a user of the Debug Bar but this also works on your Query Monitor!!

If you work with post meta like added by Advanced Custom Fields you certainly should use this plugin to extend the debugger of your choice. Choose “Debug Bar” or “Query Monitor”!!

Works with Query Monitor, too!!

Adds a post meta panel for displaying all of the post meta.

Debogger vs Debug Bar Post Meta