Outside North America the car will have a weight plate specifying the maximum gross train weight of the car plus trailer and the gross vehicle weight of the car itself. Subtract the one from the other and you have the maximum towable load. However, I do not know whether VW's for the North American market have a similar weight plate.
Towloads are specified under different conditions in Europe to those in America, American ones being more severe, hence lower limits, so it is quite likely that plated values may differ. To my knowledge, US figures are based on towing ability at high altitudes and constant hill climbing, not just a series of hill starts, as is the case in Europe.