Trouble starting jack

From Arnout Engelen

jackd -d alsa
jackd 0.100.0
Copyright 2001-2005 Paul Davis and others.
jackd comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details

JACK compiled with System V SHM support.
loading driver ..
creating alsa driver ... hw:0|hw:0|1024|2|48000|0|0|nomon|swmeter|-|32bit
control device hw:0
the playback device "hw:0" is already in use. Please stop the application using it and run JACK again
cannot load driver module alsa
no message buffer overruns

'mpg123' doesn't work, but 'aplay' does.

This seems to have been due to ESD being running. I wonder how I could've found out about this quicker, is there a sort of 'lsof /dev/dsp' for alsa?