today i was asked to change the settings on one of our customers server to 100mbs full duplex. but i had no idea how to do this. but after seareching i found the answer. you see, i have red hat fedora core 7 on my dedicated server which ran one of our wallpaper websites. so to check and confirm we are running 100mbs full duplex instead of 10mbs half dulpex, all i had to do is send this command:
ethtool eth0
now as you can see, my interface nic is eth0 so when i run this command this was the output: 
        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: 0
        Transceiver: internal
        Auto-negotiation: off
        Supports Wake-on: pg
        Wake-on: g
        Current message level: 0x00000037 (55)
        Link detected: yes
i got lucky because i didnt have to change anything because as you can see, the duplex mode is already set to full and the speed is already configured to 100Mbs . so this unix command is good for how to check speed in linux. also if you need to change the seetings you can still use this utility, this will help in changing or modifying the settins on your eth0 or eth1 for example. but if you need more help just send this command for the manual:
man ethtool
hope this helps