How to know if metacity is the window manager?

["Nigel Tao" <nigel tao gnome gmail com>]

Quick question - is there some sort of definitive X property or
environmentment variable that tells me what the window manager is,
whether it's metacity or sawfish or compiz or whatever?

Context: I have a program that would like to respect metacity's gconf
settings for e.g. focus-follows-mouse, *provided* that metacity is
indeed the window manager.