You can't really, unless you examine the HTTP headers if that is what you are using.
Its unlikely to me any cable connection can achieve that sort of speed (sustained). 10Mbps line is a 10Mbps line. And you're right, when you use browsers to download something, it has already begun downloading when you are selecting where to save the file. That is why downloads seem quicker at the start since the download rate calculation is just bytes downloaded / time, and since part of the file was already downloaded when you were selecting where to save the file to, the rate seems higher at the start.