Using a video sales letter or a webinar, what's the best way to track how much of that video someone has seen to know if they actually reached the point where they heard the pitch?

I just had a quick question about using a video sales letter or webinar. Let's say I put it on my thank you page for my lead magnet, so someone comes in, they opt in. They go to the thank you page and then I want to give them some trainings with a pitch that follows in video form. What's the best way to track how much of that video someone has seen to know if they actually reached the point where they heard the pitch? Depends on what you're using...