I have tried googling this, and I am trying to figure out if I have a 98 or the 98.5.
This would help immensly as I am trying to get a repair guide for the truck and it gives me the option of VIN 6 or VIN D.
It is the 24 valve motor if that helps, and i am new to the diesel world.

 

VIN 6 is 24v and is a 98.5
VIN D is 12v and is a 98

 

What Benjamin said. It's also the 8th digit of the VIN.

Also, inside the driver's side door should be a sticker. This sticker has a build date on it. Trucks built after 1/1/1998 are 98.5.

 

open the hood. A 24V will have a plastic valve cover and say 24V cummins in it.

 

All good ways to tell

 

I always remove the valve cover, or covers, and count my valves. 1 valve ah ah ah. 2 valve ah ah ah.