How to determine session type

Determine session type (X11, Wayland).

Directly determine session type using $XDG_SESSION_TYPE environment variable.

$ echo $XDG_SESSION_TYPE

wayland

Determine session type using systemd login manager.

$ loginctl list-sessions

SESSION  UID USER   SEAT  TTY 
      2 1000 milosz seat0 tty2

1 sessions listed.

$ loginctl show-session 2 --value --property=Type

x11