In my experience copiers, fax machines, "file server" appliances, and other embedded systems are usually the last hold-outs for WINS -- If all of your workstations are joined to the domain you can probably count them as safe (beware of badly-written legacy applications though).
When you have found and killed all the things you can think of it's time to check your perfmon counters for WINS queries -- theoretically there should be zero new WINS queries at that point. If you're seeing queries still the only way I know of to track them down is a packet sniffer (I don't believe there's a way to log who's making WINS queries).
For a small number of residual queries the usual (and more efficient) way of dealing with it is to tell everyone "On
this date at
this time WINS will be decommissioned", then pull the plug and see what breaks. If you did a good job locating and reconfiguring stuff leading up to the deactivation date you usually only have a few minor things to fix.
(If something major breaks obviously you should be ready to turn WINS back on until you can fix it!)