How can you tell if its worn? I dont think mine is just cerious really...mine hasnt been changed but I still only have 68,000kms on it (not miles) you usually change around 100,000k.
or are you talking about the serpentine belt that runs all the accessories? if your talking about the belt, if you see any dry cracks in it / abnormal wear i would replace it
I changed my timing chain when I did my heads and cam, that was at 109,000 on the stock chain. There wasn't much play in it either. I just though it safer to put he SVO timing chain on while I was in there...a bit of insurance so to speak.
I havnt seen one go out yet, (not to say they dont), I had 200000 miles on my old 95 Gt changed the HG and put a new chain on then, it really didnt need it though, just insurance like stated earlier.
If you're talking about the serpentine belt, then some cracks going across it are okay. But any cracks running along the length of the belt are bad. If you mean the timing belt (which is actually a chain), you'll have to pull the balancer/waterpump/timing cover all off to inspect it. I'm throwing on a FRPP steel double-roller when I do my heads/cam this weekend.