This information is available in a couple places. You can click on any reviewer on the right side of the screen for a display of their activity, or open up the info bar to view all invited people's activity at the same time.
The information will show if they have viewed the media asset, commented on it and approved it if (if they've been asked to).
You can also view 'Media asset info' on the Project screen. This show's activity for everyone currently involved in the video or image.
This view can be accessed by opening the options menu on any media asset in the project screen.