With truck and trailer attached and WDH on, full tank of gas and all pax on board plus your stuff.
1. Weigh front axle of truck
2. Weigh rear axle of truck
3. Weigh trailer axle(s)
4. Add them all together = GCVW gross combined vehicle weight.
Rear axle of truck should be slightly heavier than front by about 150lbs
if your WDH is setup properly.
if not follow procedures to adjust hitch (good info in this website)
then start with step one again
if GCVW is not exceeded and WDH is adjusted properly everything else usually works out.
You can weigh each axle of the truck without trailer, then add together this gives you the trucks GVW always good info to have. If you are close to your truck and trailers GCVW you might find you are slightly over your trucks max GVW but its not that big of a deal, just depends on what you are comfortable with.
you may also exceed the max rear gross axle weight slightly. I recommend d or e rated tires and some good mono tube shocks when at or near GCVW.