You could try an online speed tester such as Speakeasy, although they're pretty unreliable and dependant on a few things aside from your maximum broadband speed. Instead, head to a server with a good connection and download a file (a few MB in size at least). This should tell you your true, maximum broadband speed.
As some have recommended, there are web sites that you can use to test your connection speed.
Unfortunately, most of these sites do not give accurate results.
If you really want to test your actual speed, you might try timing how long it takes to DL Mac OS 9.2.1 update. It is a rather large file -- about 82MB.
By timing how long it takes to DL the file you can then compute the throughput of your connection. Granted there are variables but if you DL the file a few times you can average the time.
BTW, with my current connection, which shows up very slow in the web site speed tests, it takes 3 minutes. My friend's FTTH takes around 9 seconds and his connection shows up as being slower than mine.
That is why I went to the DL method.
Again, no test is entirely accurate.
I would be curious as to your DL times for Mac OS System 9.2.1 verses your advertised connection speed.