The sprockets are probably a better indicator in this case, and the sprockets and chain should be replaced as a set.
You have to know what you're looking at... wear on the sprockets shows on the rear side of the driving (front) sprocket teeth and on the front side of the driven (rear) teeth. Instead of a symmetrical "U" shape, the worn side will be exaggerated. If the sprockets look ok, there is a measurement for chain wear. First you need to examine the chain for damage, loose pins, or excessively tight or loose joints. Then the manual says (with the chain guard and sprocket cover removed) you need to hang a 20-40 lb. weight on the bottom of the chain and measure the length of 20 links on the top side, from the center of a pin to the center of the 21st pin away from that. The service limit is 321 mm. Measure several different sections of the chain...
Or at 20k miles you could just replace the chain and sprockets and put your mind at ease.
I changed mine somewhere before 20k - it seemed like a good thing to do at the time because the wear on my sprockets was very evident. Some guys run them till they look like a saw blade....
