Load up the truck and trailer as if you were going on a trip. Full fuel, propane, fresh water however you normally do, etc.
At the truck stop, Stop. Disconnect the weight distribution bars (lay in truck bed or leave them hang). Pull onto the scale. There are three platforms. Your front axle will be on one, the rear axle on another, and the trailer will be on the third platform. When they tell you they have the weights, pull off the scale, to some corner of the lot where you can drop the trailer for a bit, and unhitch. Pull back on the scales and weigh again as before. When you pay and collect the two weigh tickets, first, you will have individual axle weights for front of truck, rear of truck, and trailer, and second, you will have a total weight for the truck and front and rear axle weights for the truck.
Take the second ticket which will have the weight of the truck alone, and take that number and subtract it from the total weight on the first ticket. This will give you the trailer weight.
Then take this trailer weight you arrived at, and subtract the trailer axle weight that is on the first ticket, this will give you the tongue weight of the trailer.
Charles