Check the sticker on the pillar inside the drivers side door for the GVWR (gross vehicle weight rating) of your truck. Take that weight and subtract the ACTUAL weight of the truck (you can weigh it at a truck stop), then subtract the weight of the passengers, fuel (diesel is 7.15 lbs per gallon), tools, luggage, hitch and any other items you may carry in the truck. The remaining number is your payload. The payload is how much more weight you can carry without going over the trucks rating.
Let's use my 2002 F-350 SRW as an example:
My GVWR is 9,900 lbs
Truck Weight: 8,400 lbs with me, my wife, a full tank of fuel and my misc tools.
I have 1,500 lbs of payload capacity remaining before I reach the rated capacity. Keep in mind that you should be mindful of the individual tire, axle and wheel ratings of your truck.
Another important number is your GCWR (gross combined weight rating). Mine is 20,000 lbs. That leaves me with 11,600 lbs of towing capacity.
Always use the GVWR of the trailer you are looking at for your pin weights and combined weights. A typical pin weight is around 20% of the trailers GVWR. A 12,000 lb trailer will have a pin weight approaching 2,400 lbs.