Use ethtool or mii-tool to detect problems with ethernet card

[root@s2 adserver]# ethtool eth0
Settings for eth0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
                      100baseT/Half 100baseT/Full
                      1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Full
Advertised auto-negotiation: Yes
Speed: Unknown! (0)
Duplex: Half
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000033 (51)
Link detected: yes

You can also change the interface settings with ethtool.

[root@s2 adserver]# mii-tool
eth0: negotiated 10baseT-FD, link ok