Yes you can do a ohm test on each of the speed resisters, but its just easier to take it out and look at it or measure it to tell if its bad. It should be apparent if it is bad.

But, I'm going to say as old as the truck is, its no doubt the resister, or/and the connecter itself as well may be fried . Get it apart and look at it, its probably already been changed before, maybe even a couple of times.