In the following example, I am testing the speed between two virtualbox debian machines, both with Intel Pro/1000 MT Server (82545EM) virtual ethernet adapters.

1. open iperf with -s option to one of the servers:

NOTE: iperf will remain open until you type ctrl+c

root@pluto:~# iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 16.0 MByte (default)
------------------------------------------------------------

2. Run iperf with -c IP against the server from point 1:

[root@sun] ~# iperf -c 192.168.69.8
------------------------------------------------------------
Client connecting to 192.168.69.8, TCP port 5001
TCP window size: 35.0 KByte (default)
------------------------------------------------------------
[  3] local 192.168.69.1 port 41076 connected with 192.168.69.8 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec  1.10 GBytes   944 Mbits/sec

On the 1st server you should see iperf writing forward (green output):

root@pluto:~# iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 16.0 MByte (default)
------------------------------------------------------------
[color=green][  4] local 192.168.69.8 port 5001 connected with 192.168.69.1 port 41076
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.0 sec  1.10 GBytes   941 Mbits/sec[/color]