It can sometimes be difficult to determine which networcard is eth0, eth1 and so on. A easy way to determine it is to use the ethtool command
ethtool -p eth0 10
This makes the network cards LED blink for 10 seconds, and this happens without interrupting the network traffic.
You do offcourse need physical access to the server to actually see that the LEDs blink :)
To install this tool in Debian you have to run the command
apt-get install ethtool