Fluctuating neg. to ground on ignition coil

Stephan D said
Is there a simple way to find out if the distributor bushings are worn?

The engine(and truck) has only 40.000 original miles, seems a little early to wear out?

Pull the distributor cap off and try to move the rotor side to side.
Should be very very little to no movement.
Dial indicator would be best as suggested Rusty.
I dont know off hand what the spec is maybe someone can chime in with a number.