You can check the current settings from the shell with the ethtool command. To see what's configured on eth0, you can do this:
# ethtool eth0
If you need to change either setting, you can use ethtool -s eth0 [ setting | value ]. So to set, say, duplex to full on eth0, you could do this:
# ethtool –s eth0 speed 100 duplex full autoneg off
It's important to note that this command doesn't set this permanently, so after a reboot it may revert. To make it permanent, add this line:
ETHTOOL_OPTS="speed 100 duplex full autoneg off"