Select Smoothing Group
Use the Select Smoothing Group tool inside the Smooth Group Editor window to select faces matching the current smoothing group.
To use this tool:
Open the Smooth Group Editor window.
Click any face with a smoothing group defined.
Note: The tool doesn't shrink the current selection, so it's best to start with as small a selection as possible.
Click the Select Smoothing Group button.
Tip: You can also access this tool from the ProBuilder menu (Tools > ProBuilder > Selection > Select Smoothing Group).