That's tricky and I don't have any great advice. If you could find some tangible piece of knowledge associated with each video, then have a quiz after some time

That's tricky and I don't have any great advice. If you could find some tangible piece of knowledge associated with each video, then have a quiz after some time, that might help. Or introduce some kind of spaced repetition. Alternatively, if you make the video categorization about what the viewer is interested in learning (that is, make the interface drive people to see videos based on what they want to learn, rather than having a hierarchical organization), then you can ask them later "did you learn X?" with some other survey like questions.