Length of the tow cannot be determined by stern view u need to see the mastheadlights for determining the length of the tow 

Well if its day time , if you can see the diamond shape , its more than 200 metres . Else its less

To be practical, determing the length of the tow from Stern view will be irrelevant as you will most likely be overtaking the vessel. The length of the tow will matter when the T&T will be in a crossing situation with respect to you. In this scenario you will be able to identify by the diamond shape displayed. Also even visually you can verify.