Looking Glass is GNOME Shell’s integrated debugger and inspector tool. It aims to be the Firebug of GNOME Shell.
You can open looking glass by pressing Alt+F2 and typing “lg” and pressing Enter, or by getting this new useful extension that will make the job for you!
You can simply click LookingGlass Button on top panel to open and close lookingGlass dialog, or press F12 key in active mode, or use Ctrl+Alt+Tab to put cursor on top panel,move cursor to LookingGlass button and use space or down to open lookingGlass.
To use this extension simply turn the switch to “ON”