* Check the sticker: Look for a sticker on the driver's side doorjamb. This sticker will list the GVWR (Gross Vehicle Weight Rating), the curb weight, and the payload capacity. The payload capacity is the difference between the GVWR and the curb weight.
* Consult your owner's manual: The owner's manual should also specify the payload capacity for your truck.
Without that information, any number I could give you would be unreliable and potentially dangerous.