Path: utzoo!attcan!uunet!husc6!bloom-beacon!EXPO.LCS.MIT.EDU!rws
From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: Is a window manager running?
Message-ID: <8811161351.AA28852@EXPO.LCS.MIT.EDU>
Date: 16 Nov 88 13:51:17 GMT
References: <3921@omepd>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 5

Window managers typically select for SubstructureRedirect
on the root window, and only one client can select for this.
So you can use success/failure of selecting for this as a
pretty reliable indicator.  The fact that not all window
managers check for failure here is a bug.