Another good indicator is to gauge the tight/loose condition of the chain and sprockets. Specifically, your chain will have tight spots and loose spots due to uneven wear of the sprockets, which become a little eccentric (egg shaped). You want to find the tightest spot on your chain and then adjust your chain to the appropriate slack. Rotate your rear wheel at least one full turn while you continue checking to see if the chain gets tighter than the point at which you originally adjusted it. With a worn out chain and sprocket set, the slack portions of the chain will be so loose that the chain will easily contact the swing arm, making a slapping sound when you ride. At this point, the chain is dangerous and should be replaced. You cannot tighten your chain to stop the slapping because it will end up being too tight in the tight spots.
Finally, whenever you replace a chain for wear, you should also replace the sprockets, or the worn sprockets will accelerate wear on your new chain.
As for the OP Edcop, if you got 30,000 miles out of your chain and sprockets, that is pretty impressive for a bike of this weight and power.