A truck’s Vehicle Identification Number (VIN) can help determine its towing capacity more accurately.
Following are the links to a few automakers' calculators to find out a truck’s towing capacity or GCWR (Gross Vehicle Weight Rating).
The Toyota Owners vehicle specification page can provide a truck’s GCWR and towing capacity numbers plus other data.
GMC provides Towing Charts for 2018 model GMC’s.
RAM Trucks has a Towing calculator to help find a vehicle's towing capacity.
Chevrolet VIN decoder can be obtained from the Chevy Enthusiasts Forum.
To compute a maximum towing capacity, check the curb weight on the VIN sticker and then add the weight of the cargo, fuel, and weight of your passengers. Subtract this number from the vehicle’s GCWR (which can often be found on a truck’s door sticker).
Factors that can contribute to a pickup's towing capability are the engine type, axle ratios, transmissions, chassis, suspension, tires and load ratings, hitch type and class and trailer type.