If you're monitoring your Internet traffic, the best measure is to visit a site like http://speedtest.net and run the test whilst simultaneously watching the graph in PeakHour.

If you're not monitoring Internet directly (you may be monitoring your WiFi for example), you can use something like Time Machine or a file transfer from one machine to another to generate the traffic. You can then use the Network tab in Activity Monitor to verify the speeds are correct.

Check out the Usage Monitoring page for more information on accuracy.