From https://surfer.nmr.mgh.harvard.edu/fswiki/FsTutorial/OutputData_freeview.
Open a terminal, move to the subjects folder:
[user@localhost ~]$ cd $SUBJECTS_DIR
Open with the following command.
[user@localhost ~]$ freeview -v CON178/mri/T1.mgz CON178/mri/wm.mgz CON178/mri/brainmask.mgz CON178/mri/aseg.mgz:colormap=lut:opacity=0.2 -f CON178/surf/lh.white:edgecolor=blue CON178/surf/lh.pial:edgecolor=red CON178/surf/rh.white:edgecolor=blue CON178/surf/rh.pial:edgecolor=red
Tip: copy this text into notepad, and search and replace "CON178" with the subject you want to view.
The fs wiki page has more details of how to navigate.
Segmentations
Double-click "aseg", and increase opacity.