Yes, I know what you are asking. (You forgot about seeing if they have their smartphones open, or a second computer open.)
Consider sitting next to the student and observing what they do. You will see what they are physically doing, but you will probably not see what is going on in their brains (e.g., are they really watching the video, or reading what is on their screen.)
Some schools have "testing centers" set up, which may be a solution.