One thing you can look for is on the rear of the cab. If it was originally a fleetside, the is no splash guard panel on the lower corner of the back of the cab. If it was originally a stepside, the splash guard is there. In the attached picture, you will see there is no splash guard. This truck did not originally come with a stepside bed. I know this because I changed it from a fleetside to a step side after I got the truck. I still need to fabricate and add the splash guards to keep road debris and other junk off of the steps.
