A lot of ways to tell.If there is a visible 1/8 in.pipe plug behind the water pump.it is a 4 bolt main.pretty easy to spot while looking in from the front of the vehicle or engine.To the best of my knowledge all truck engines were 4 bolt mains.A lot of people think the 4 bolts are stronger,and this is not nescisarilly true.The 4 bolts use four small bolts in each main,while the 2 bolts use 2 larger bolts.So there is really not much difference in strength.
