You can try the new "Collect Client Statistics" feature in Performance Advisor 4.0. It will report on every server (IP address) that is accessing the NFS export at that time. In addition, it will show how many operations each client is performing at that time. You can run this on-demand or schedule it to run on a regular basis.
You can access this in the NetApp Management Console for a GUI. You can also run this from the DFM server's CLI.