1/2 ton frames are wider at the rear than at the front, 3/4 ton frame rails are basically parallel front to back. Check the frame width at the rear. If the width is about 45 inches it is a 1/2 ton whether it is a short bed or long bed. Only GMC had long bed half ton so if you are looking at a Chevy long bed it is 3/4 ton.

Mar-Ktech