Backdrop has a Theme Debug mode that can be enabled and disabled via the a hidden setting. Theme debug mode can be used to see possible template suggestions and the locations of template files, as HTML comments in the markup of the page.

You can enable the setting using the following PHP code snippit:

config_set('system.core', 'theme_debug', TRUE);

And disable using:

config_set('system.core', 'theme_debug', FALSE);

Alternatively, If you would prefer to use a button to enable/disable theme_debug mode, the contributed module Devel provides an option to enable the theme debug mode through the admin interface. Once you install and enable the module, navigate to Configuration -> Development -> Devel settings, enable the Enable theme debug option under the `Core system devel settings` section and save the configuration.