if you are looking for which eth# has a cable plugged in try:  sudo ethtool eth0 | grep Link

Given the aforementioned, if my eth0 NIC interface is being used, the following will result:

[root@localhost ~]# sudo ethtool eth0 | grep Link

       Link partner advertised link modes:  10baseT/Half 10baseT/Full

       Link partner advertised pause frame use: Symmetric Receive-only

       Link partner advertised auto-negotiation: Yes

       Link detected: yes

NOTE:

The only problem with the above ethtool output is that it will not detect connected cable if your network interface is down. Consider a following example:

# ethtool eth0 | grep Link

       Link detected: yes

# ifconfig eth0 down

# ethtool eth0 | grep Link

       Link detected: no

# ifconfig eth0 up

# ethtool eth0 | grep Link

       Link detected: yes

[8:07]

So, recommend to add ifconfig eth0 up before ethtool