How can one tell if more teamsters are needed?
It is often as much a function of pathing and staffing levels as anything. One good indication is if your factories have excess finished goods sitting around and not enough of their required inputs. As I said however sometimes traffic congestion and long drive distances between places can do just as much to reduce the effectiveness of the teamsters. My advice is just to eyeball it based on having little output sitting at the factories and a healthy supply of inputs.