[cybergomer]

There are several ways to tell. First, inside the cab, the 4x4 control is either a shifter or a shift on the fly design. I'm not positive on the options for an 88, most likely would be a shifter on the floor.

If there is nothing there, there also may be locking hubs on the front wheels. It will be a dial in the center of the front tires, most likely saying "free" and "lock", lock meaning engaged 4x4 and free being just 2wd.