This command seems to be useful:
ls /usr/bin/*session*
- in GNOME returns /usr/bin/gnome-session (and more)
- in MATE returns /usr/bin/mate-session (and more)
- in LXDE returns /usr/bin/lxsession (and more)
- in XFCE returns /usr/bin/xfce4-session (and more)
- in JWM returns /usr/bin/icewm-session (should be jwm-session, not?!)