The simplest way is to know the total weight carrying capacity of your two rear tires and then weigh your rear axle when loaded. The maxiumum load capacity of each tire at maximum inflation pressure is molded on the sidewall of each tire. It is approximately 3,000 lbs. per tire or 6,000 lbs. for the rear axle. Obviously, if you routinely exceed the capacity of the rear tires you will experience tire failure and possible loss of control.