To tell if your chain is knackered you'll have to test for "tight spots" by (preferably) putting it on a paddock stand and slowly spinning the wheel round to check it every couple of inches. If you find a tight spot and at the same corresponding point you can pull the chain away from the rear sprocket by anything other than a very small amount, then it's time to change your chain and sprockets.
I'm in this dilemma at the moment.
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