Ask HN: How do I proove my ISP is throttling speed

Since last few weeks none of the videos in Yourtube are getting streamed in real time, even with the low quality (360p, 480p etc.) However speedtest showed its still 20mbps and no other website has loading issues. For example the same video in two different streaming site https://www.youtube.com/watch?v=jAmY4m7FcKs does not stream and buffers every few second but https://vimeo.com/channels/staffpicks/135806831 buffered completely in 30 seconds. It is least likely YouTube is having a server problem, help me understand if ISP is throttling Youtube specifically.