Check your ubuntu desktop session

• To check the current desktop session name:

  echo $DESKTOP_SESSION
  

  It will return the name of the current desktop session. e.g. ubuntu-2d (aka. unity 3D), gnome-shell, gnome-classic.
• To check the default desktop session:

  cat /etc/lightdm/lightdm.conf
  

  It shows the default session: e.g.

  [SeatDefaults]
  user-session=ubuntu
  greeter-session=unity-greeter
  

• To list the available session definitions in /usr/share/xsessions/:

  ls /usr/share/xsessions/