in Terminal, do ifconfig -a, and look under en0's "media" line. Mines' in 100 half-duplex mode.
[localhost:~] dorfman% ifconfig -a
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000
en0: flags=8863<UP,BROADCAST,b6,RUNNING,SIMPLEX,MULTICAST>...