How long you stay on a site or focus on the page with the ad visible can determine this somewhat. If you quickly scroll by once the ad becomes visible and this

How long you stay on a site or focus on the page with the ad visible can determine this somewhat. If you quickly scroll by once the ad becomes visible and this behaviour is consistent with other pages with the same ad, then you're probably not showing a good ad. Additionally advertisers do have surveys where users are asked about this (usually not many but sufficient to get a first guess). On youtube the example would be the ratio of people clicking the ad, people letting the ad play out and people skipping it (and then likely also measuring how long it takes for them to click skip once it's available).