Basically, if you look below your XFCE application menu and see any
application with aname which starts with a "k" it will depend upon KDE
libraries. Examples of such applications are:
k3b (for burning CD, DVD or BluRay)
Konsole (yet another terminal)
KMail
Konqueror (web browser and file manager)
Krita
KPatience
KStars
KCachegrind
KCalc
However, there are also applications which does not start with "k" which
depends upon KDE libraries. Exampels:
The hated nepomuk
calligra
Amarok
Granatier
If an application below the "help" menu has a choise "About KDE" it will
for sure depend upon KDE libraries.
However, all the applications above are in the slackware*/kde directory
of the installation media. So yes, you will save a lot of disk space by
not installing them, but you will also miss a lot of applications from
your XFCE menus.
regards Henrik