- connect your computer to another Internet connection (other ISP, other modem), and check the max downloadspeed
- connect a fresh, clean computer (you could live-boot Ubuntu) to your own Internet connection, and check the max downloadspeed
That way you can find out by deduction / reduction / induction
Oh, BTW, you're not using Wifi, I hope? If so, first test is to use a wired connection.
HTH

