Checking Duplex

To check the duplex status from the command line issue

mii-tool
This will return a result like
eth0: negotiated 100baseTx-FD, link ok

You can also issue mii-diag for more detailed information
Using the default interface 'eth0'.
Basic registers of MII PHY #32:  1000 796d 001c c912 0de1 c9e1 000d 2001.
The autonegotiated capability is 01e0.
The autonegotiated media type is 100baseTx-FD.
Basic mode control register 0x1000: Auto-negotiation enabled.
You have link beat, and everything is working OK.
Your link partner advertised c9e1: 100baseTx-FD 100baseTx 10baseT-FD 10baseT.
End of basic transceiver information.