I do a variation of
@ChrisLegend does.
In your analytics, go to "Audience Retention". It will default to show you average view duration, but you can switch it to average percentage watched.
I go a step further. From the Audience Retention analytics, you can then also see, by each video, what the retention is, and if you click on a video from the retention screen, it'll take you to another page where you can see by second what your audience retention is. I look for when the video dips below 50%, and around that time, I put in an annotation and card to another video.