Codepress

It shows a tab with diagnostic info about your site and settings. You can use this plugin when you have a support issue.

Admin Columns Diagnostics

This plugin assists you with support for Admin Columns.

gilles66

Need to know which template is used by WordPress to display your pages in the front office ? 
This plugin simply does this.
Show the name of the php file of your theme used to display the current page.Need to know which template is used by WordPress to display your pages in the front office ? 
This plugin simply does this.
Efficient and very easy to install, it will show you in the admin bar the name of the php file currently used to display the current page. 
The color of the text is different regarding the origin of the template(the theme, a parent theme, or a plugin) 
(icon author :http://www.megaicons.net/iconspack-1096/45043/)

which template file

Show the name of the php file of your theme used to display the current page.

Admin Columns Diagnostics vs which template file