I would run into this all the time from our cabinet and cloud customers in Level3. The question or accusation was always the same. "How do I know I am getting a 100Mbps or a 1Gbps handoff from you guys?" "I just tested and I can only get X amount of bandwidth, just look at my Cacti graphs and you will see I am not getting the full amount I am paying for".
So after a short explanation of how a single connection to their server will never fill up the pipe, I would have them run the test, or I would login to their server and run the test for them, while having them monitor the Cacti graph. Cachefly and a few other CDNs have a 100mb.bin and 1000mb.bin files that are not compressible and are not affected by WAN accelerators, etc... So, the trick was to start clicking on the files for download to the server as fast as you could, and watch the pipe fill... most often, their RDP, or whatever remote solution they used, would get so lagged, they thought the server was dying. The files would finish downloading and the remote connection would be normal again.
Me: "What does Cacti show now?" Them: "Thanks, I am going to use that test on everything from now on, very cool". I would look at the Cacti graph afterwards and you see what you expect, a steep 90 degree hill of all green for a few minutes and then back to the 1-2Mbps sustained that they are used to seeing. There may be better ways, but I never had a customer argue with me after using this method and seeing the results in Cacti.
Real Programmers don't eat quiche. They eat Twinkies and Szechwan food.