You cannot determine the precise towing capacity without checking the sticker located on the driver's side doorjamb. This sticker lists the vehicle's GVWR (Gross Vehicle Weight Rating) and the maximum towing capacity for that specific truck.
To find the *approximate* range, you can consult a 2004 GMC Sierra 1500 owner's manual or a reliable online resource that specifies towing capacities for that year and model. However, the information on the doorjamb sticker is definitive for *your* specific truck.