mii-tool

mii-tool can be used to check the speed at which your ethernet card is operating at under Linux. For example:

eth0: negotiated 100baseTx-FD flow-control, link ok
product info: Intel 82555 rev 0
basic mode: autonegotiation enabled
basic status: autonegotiation complete, link ok
capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
advertising: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
link partner: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control

Above tell us the network link over at interface eth0 is working at 100baseT speeds and FD means full-duplex.

Cheers,
Steve