First locate a commercial truck scale in your area. Look in the yellow pages under truck stop. The scale operator will know how to weigh your rig.
Generally it goes like this:
Drive on scale with front wheels of the truck to get the front axle weight.
Pull forward till rear axles are on the scale to get the loaded truck weight.
Pull the trailer onto the scale to get the total weight of the rig.
Pull off the scale and unhitch the trailer and get the truck alone weighed.
Total weight - truck weight = trailer weight
Loaded truck weight - truck weight = payload on truck (hitch/pin weight)
Unless you are really into fine details, just find a scale at a truck stop or other location that has the three part survace. The front will weigh your tow vehicle's front, the middle the rear axle and hitch weight and the third the wheels under the trailer. They will be separate in a printout that also includes the total.
That's all there is to it and enough information to know if you're pretty even, overweight or what.
The best way is to first put your steering axle on the scale and get a weight. Then add one axle at a time, getting the new weight. That will give you the most accurate picture.
That said, I last did it by first weighing the truck (with trailer attached) then weighing the truck trailer combination. I then returned a few days later and weighed the truck without the trailer. This method won't give you the percentage of trailer weight being transferred to the front truck axle, nor tell you if you have an equal load on your trailer axles. I does tell how close to any of your limits you may be.
On the scale
For weighing a motorhome, position it with all wheels on the scale for the GVW. Be sure to add passengers' weight to the GVW figure. For the front and rear axle weights, move the motorhome so that just one axle is on the scale for each reading. For side weights, position the side not being weighed off the scale but as close to the scale edge as possible.
Trailer weighing is slightly more involved. Before unhitching the trailer, weigh it and the tow vehicle to determine the Gross Combined Weight (GCW).
The GCWR, which is primarily based on engine type, is found in the towing guide of the tow vehicle manufacturer. Positioning the tow vehicle so its rear wheels are just off the scale provides the trailer's axle weight.
Before recording a travel trailer's axle weight, release the spring bars and remove the sway control. Since all trailer axles are equal as far as the weight they are designed to carry, divide the total axle weight by the number of axles to get the weight each axle is carrying. Unhitch the trailer and weigh it alone to get its GVW.
Calculate the hitch weight by subtracting the axle weight from the GVW. For proper tracking, stability and ease of towing, a travel trailer's hitch weight should range from 10 to 15 percent of the trailer's gross weight. A fifth-wheel trailer's hitch weight should be approximately 25 percent of the GVW. Side weights are obtained in the same manner as for motorhomes.
A certified public scale provides accuracy within 20-25 pounds.