Follow this link to see how Netflix rates your ISP
https://ispspeedindex.netflix.com/country/us/?small=Tru&kbid=147358
I guess they can see how fast their content is streaming to you. Netflix content buffers so they will be able to work out the speed by seeing how much they can buffer in a given amount of time.
EDIT: Actually, here is the explanation:
https://ispspeedindex.netflix.com/about/
They just measure the speed of their content getting to the user. Remember, Netflix is variable rate and they have numerous different streaming rates for their content.
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