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
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