The fuel weight should already be included same as that 150# driver. Best to get a scale ticket.
Fuel should be included in the stickered payload number. Payload is simply the empty truck with full fluids subtracted from the GVWR. There is NO weight included for a driver. A common misconception has people believing that payload includes 150lbs for the driver. Not true. The TOW rating includes 150lbs for a driver. 
As mentioned, the most accurate way to determine your exact payload is to fill the tank, load the passengers and anything that will be carried in the truck and head to the scales.