The easiest way would be to try calling their customer service and ask them. Most hard-wired internet providers in the first world don't care how much data or what type it is, within reason of course. If they do though, they are generally up-front about informing customers.
The next best way to test this is to check other video streaming sites, such as Vimeo, Revision3, and even Facebook to get a baseline. Perhaps even upload a test video of your own if you want to be scientific. After that, you should use a VPN to access those sites. Make sure the VPN encrypts your traffic. This will make all the data that goes over the wire look like gibberish, and it would all look the same, with no noticeable difference between streaming videos, or anything else.
When on the VPN, how does your streaming look? Granted, a VPN adds an extra few hops for your data to go through, but if the ISP is filtering based on the IP or the content of the Data, you will be able to notice significantly better streaming!