Title says it all. If you want to check (or set) your ethernet NIC configuration or status in Linux, people used to use mii-tools. But there’s a lot more powerful and modern tool that obsoletes it: ethtool
You can install it with your favourite package manager, if it’s not already present in your system.
In Debian/Ubuntu, you can issue
# aptitude install ethtool
Here it is an example:
# ethtool eth0
Settings for eth0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: g
Wake-on: d
Current message level: 0x000000ff (255)
Link detected: yes