If you work remote and want to test if the link is up or not on your linux interfaces use mii-tool.

If the thing is not already installed on your machine, if you run debian simply use apt-get install mii-tool . After the tool in installed just type mii-tool in your console.

Example:

server1:~# mii-tool
eth0: negotiated 100baseTx-FD flow-control, link ok
eth1: negotiated 100baseTx-FD, link ok
eth2: negotiated 100baseTx-FD, link ok
eth3: negotiated 100baseTx-FD, link ok