Please correct me, but to my knowledge there is no mechanism in place to warn users when the Trinity compositor will not work. There are two conditions that I am aware:

* An existing xorg.conf contains the following:

Section "Extensions"
  Option "Composite" "false"
EndSection

* A packager purposely disables compositor support such as proposed in enhancement request 1289: -DWITH_XCOMPOSITE=OFF.

Currently with either condition, the user is allowed to enable the compositor, KControl->Desktop->Window Behavior->Translucency, and there is no feedback that this is an exercise in futility, which likely will confuse most users.

What is the preferred method for informing users why their efforts will not succeed, or to inform them that the options are unavailable?

Darrell