I know that, because on the channel traffic/engagement statistics page it will tell you the total engagements, as well as other various stats such as average watch time, and repeat views. The more people come back, and longer they watch, I'd think the algorithm favors and recommends the video more. However, even if you just watched it for 1 second, you still engaged with it, so I'd assume that would count as a view. The thing I don't know is how they define an engagement. Whether it is following a link/thumbnail/imbedded url to the video, or actually playing it.