i check for that by marking from the center hub to the outer ring the belts ride on with a grease pen, then start it and run the steering to full lock hard a couple times, shut it off and check the mark to see if the inner hub is slipping inside of the outer. the problem is the rubber shrinks that's between the hub/ring

Quote Originally Posted by Bill R.
lucky it spits it out the bottom and goes rolling across the road as your driving... If you're not so lucky it makes its way through the fan and through the radiator...its usually pretty obvious when the harmonic damper is starting to go bad though, you can see it wobble long before it goes completely