There should be a payload sticker on the driver's door. It's usually white and yellow and it will list your payload capacity. Subtract the weight of your passengers and gear and that's what's left for your hitch weight.
Or, you can truck the truck fully loaded with fuel, passengers, and gear to a scale and weight it. Subtract the scaled weight from the GVWR and that's how much hitch weight you can carry.
Scales can be found at truck stops, quarries, feed mills, scrap yards, and land fills.
While your at the scale, you might as well weigh the front and rear axles separately. Many people, including DOT officers, go by axle and tire weights rather than GVWR. If you subtract your fully loaded rear axle's scaled weight from the RAWR on your door sticker that will likely give you a bit larger number for your tongue weight.