Jack the front end up and spin one tire; is the other tire trying to spin? Yes = Posi, No = Open Differential (no posi). It is really rare to have Posi in the front end of our trucks, because they are driven in 2WD most of the time and the Posi would make it very hard to turn the front wheels to the right or left. This is due to the Posi action trying to keep the wheels spinning at the same rate. This defeats the differential. Posi in Jeeps or other vehicles that are used off road mostly, are more common.
Dave