Measuring the network speed is not quite easy, since there are bottlenecks outside the network interface and your machine. Cable problems, poor choice of switching gear, and congestion on the line or the remote machine you want to test against can all reduce the throughput of your network interface. In addition, protocols like SSH or HTTP have additional processing that may need to be done that occupies the CPU and reduces throughput.
If the machine you are testing is going to be a server, you can create a small group of client machines on a private network. These machines should have the same type of network card and OS revision. This will create a stable baseline of testing.
Depending on the networking application you will be using, there may be applications that already exist to automate this testing for you. Programs like webbench can coordinate the clients talking to the server, and be able to read the performance of the server in terms of pages per second.