Ethtool

ethtool is a program to find your NIC's type and abilities. When everything is right, it looks about like this:

$ ethtool eth0
Settings for eth0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Advertised auto-negotiation: Yes
        Speed: 100Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: umbg
        Wake-on: g
        Current message level: 0x00000001 (1)
        Link detected: yes

Link detected: yes means

• your NIC is up (ifconfig eth0 up)
• a cable is plugged in and you have a working connection to your network switch (or to another node).

Problems

The line

Link detected: no

can just mean you have not done an ifup on your network DevIce.