If the laboratory has a deadline, you should establish with your demonstrators a system for determining which students work needs marking. One of the most widely used schemes involves the students writing their name and machine name on the white board when they are finished, thus forming a ``marking queue''. The demonstrators can then target these people in turn. This is the recommended approach, particularly in large laboratories. Once you have agreed the protocol with your demonstrators, you can announce it to the students.
Your demonstrators should each take a ``Generic Marking Sheet'' to record the marks they make during the session. Please ensure they fill in the details at the top of the sheet: course, marker's name and date, otherwise you, and even worse, we, will have a harder time tracking down the inevitable mistakes later.