Usually you have a tool comming with your wm... For kde it's "kdesu"... It launches a command as root, but setup display so it can use the "user space" xserver (it's a simplification).
You can use kdesu even with other wm comming with slackware, of course, if you've done a full install...
Code:
bash-4.1$ kdesu thunar &>/dev/null &
(You can ommit the redirection and the background parts if you want some verbose/debug...)
EDIT: I haven't tried, but you might even be able to do something like:
Code:
bash-4.1$ kdesu terminal &>/dev/null &
... and launch a 'root' terminal, being able to launch any graphical tool from root...
WARNING: I haven't tested that, it needs to be confirmed.
Bests.