How do I find out whether a network connection is full or half duplex?
You can use mii-tool :
'FD' is full duplex. 'HD' would represent half. For those who haven't seen this, it's because it generally represents a network problem -- you should always see and expect full.
If you don't have mii-tool installed, it is in the net-tools package:
I believe it is included with a base install, however.
In some cases, dmesg will be sufficient.