how to determine current X focus?

[michael <cs@networkingnewsletter.org.uk>]

Is there a tool that will o/p the current X application that is
accepting input ('focus')? It appears my problem with (intermittently)
un-responding USB mouse/keyboard (see 18 Dec 2007 posts) may be due to
loss of focus [1] and I wish to investigate this further.

[1] I come to this conclusion since if in Gnome I turn off Gnome
(metacity?)'s "select windows when the
mouse moves over them" option (and thus also turning off auto-raise) it
*seems* to make the problem go away

Thanks, Michael