I received a new answer right now

echo $WAYLAND_DISPLAY (if it outputs "wayland-0" or something it's a Wayland session),

or echo $XDG_SESSION_TYPE

I don't know how and why these variables should be set on GuixSD but people on the fediverse don't know that I asked on behalf of you ☺

Il giorno ven 4 gen 2019 alle ore 12:52 Catonano <catonano@gmail.com> ha scritto:

another answer arrived rigt now

If you open xeyes, the eyes will not move if your pointer is on top of a Wayland window.

Also, the combination

Alt+F2

and then

R + return

restarts Gnome if it's on X. It doesn't if it's on Wayland