Load it up the way you normally travel including stuff in the truck along with the passengers who normally travel. Head for the CAT scale and drive on in such a manner that the truck's axles are weighed separately along with the two trailer axles together weighed separately from the truck.
Then drive out to the parking compound and drop the trailer and re-weigh the truck. Do the math, the difference in the rear axle will be the pin weight.
Kudos to you for wanting to know your true actual weights.