Well, startkde is pretty much a dead give away for KDE. I believe gnome-session is the GNOME equivalent. I'm not positive, but I believe one of those processes will always be running if the user is logged into KDE or GNOME.
Other than that, it can be hard to tell. If you start up Konqueror or Nautilus, for example, they bring up half the desktop with them (gconfd, dcop, etc), so I don't think there are too many other processes that you can rely on.