A colleague of mine demonstrated this for me yesterday while we were troubleshooting which physical NIC out of 8 in a rack mount server matched up to which vminc (0 - 7) in vSphere. Tougher than you might think when vmnic6 and 7 mapped to the onboard physical NICs 1 and 2!
Using the command line tool ethtool you can make each vmnic blink for a specified period of seconds allowing you to identify which port on the back of the server it maps to. So to make vmnic0 and 4 blink for 5 seconds, use the following:
ethtool -p vmnic0 5 ethtool -p vmnic4 5