from Irongeek.com

Finding Promiscuous Sniffers and ARP Poisoners on your Network with Ettercap

Most of you are familiar with using Ettercap for attacking systems, but what
about using it to find attackers? This tutorial will cover using Ettercap to
find people sniffing on your network. The plug-ins we will be using are
search_promisc, arp_cop and scan_poisoner.  Have fun detecting network
sniffers.

If the embedded video below does not show RIGHT click here to save the file to your hard drive.