Start with the gtk3 handbook – the first two chapters. Then read up on the Gtk Inspector and understand how to start it.

To start a GTK3 app in inspector debug mode, you would:

GTK_DEBUG=interactive <app>

To start it with xfce4-panel, for instance, you need to first quit the panel, then start it in GTK debug mode:

xfce4-panel -q && GTK_DEBUG=interactive xfce4-panel

When running xfce4-panel like this, you will get many inspector windows popup–you’ll need to find which one belongs to the element you are looking for. You can use one of two methods:

• Double-click the tree entries to expand them and look for the widget name in the list (e.g. clock, pulseaudio-plugin, etc)
• Click the locator in the top-left of the inspector window and then hover your mouseover every plugin until its background changes colour–then you have found it.