This information is available in two places. You can click on any reviewer on the side of the video for a display of their activity.
The bar below their name shows if they have viewed the video, commented on it and approved it if (if they've been asked to).
You can also view 'Video info' on both the Review screen and the Project screen. This show's activity for everyone currently involved in the video.
This view can be accessed by opening the options menu on any video in the project screen, as well as a button in the top right of the screen on the review screen.