Meddling with X displays over SSH

Since I use SDDM, the Xauthority file is located in a randomly-named file in /var/run/sddm. To meddle with local displays, you can simply:

$ export DISPLAY=:0
$ export XAUTHORITY=/var/run/sddm/...

Now you can do stuff like xset dpms force off to turn them off, or open programs on them, or whatever.