The next version of OptiGUI (2.2.0-alpha) will add a very basic inspector. This adds a key bind to copy an OptiGUI INI formatted text containing the container ID and the selectors to the clipboard.

Preview

Pressing the key binding on any supported screen will copy the INI, and show a toast like this:

Key binding

The default key binding is F12 (just like opening the inspector in the browser), which can be changed in Minecraft settings.

To add a GUI or not to add a GUI

Adding a GUI was the first idea I wanted to do, but making a GUI takes time. Making a GUI with an unknown framework takes even more time.
Correct me if I'm wrong, but I don't think there's a need for a fancy in-game GUI to display some text. Inspector is meant to be used by resource pack creators, [and they probably want to use the inspected data (paste and edit it). I dunno, this seems logical].