Pritham Shetty, server guru of Macromedia, has provided some scripts with which you can test a user's bandwidth. I am providing a functional version of it here on my site, slightly modified. Give it a go below.

Please note that this test DOES NOT determine the speed of your connection. Instead it basically determines what kind of datarate you are getting between your computer and my server. Keep this in mind because connection or bandwidth problems on the server will reflect badly on the results even if your connection speed is very good.

Sources are here.