How to identify ISP speed throttling

Hi,

I have dell xps 15, using it since last 4.5 years. I recently got a fiber optics broadband connection, got a new netgear AC router too. I have noticed that I am unable to watch streaming videos since last 3 weeks. In the starting, it was less frequent, but now its like I cannot watch videos in many sites. I would like to find the cause; if its the system, router or the ISP itself. Because whenever I check the internet speed, it shows 30mbps most of the time, almost always it is above 5 mbps, which should be reasonable speed for streaming. How do I identify if the ISP is throttling streaming sites to save their bandwidth?