I'm aware of how important they are, but have been coming up empty in my searches to determine if they're worn to the point of needing replacement. The service manual just says to inspect and replace as necessary (as a set). I don't see anything I can measure to determine if there is any wear? Obviously there's a point somewhere in between waiting for it to fail and replacing it every time I ride... :-)
I got this advice from this website:
Go to your rear sprocket and pull straight back on the chain. If your chain pulls away from the sprockets by much, it is probably stretched out. If the chain does not pull away and stays right on the sprocket, then the chain is not stretched out yet. Also, if your sprockets no longer look like points but a bunch of little hooks you need to replace it all. Another wear indication is tight links or kinks in the chain. This is caused by a lack of lube that has caused links stick.
The most common way to know if your chain needs to be replaced comes from measuring your slack. As a chain wears, grooves are cut into the pins which gives the illusion of chain-stretch. No chain wears perfectly even so every chain will develop a variance in the chain slack measurement. If the variance between the tight spots and loose spots of the chain become too great, you'll want to replace the chain. If the chain is not replaced at this point, the variances in slack will put a lot of stress on the sprockets. Replace the chain early and most times you can run two chains to one set of sprockets.
According to both the Chilton workshop manual and the Clymer workshop manual for Norton 750 and 850 twins, remove the chain from the motorcycle. (I temporarily connect a spare chain to the rear chain to make reinstalling the chain easier.)
Lay the chain on a flat surface and compress all the links together. Measure the length of the chain and write that measurement down. Now stretch the chain, re-measure it, and write that measurement down. Chilton: "The maximum acceptable extent of chain wear is about 1/4" per foot. If the chain stretches more than this amount, the hardened surfaces on the bearings will have worn through and the chain should be replaced. If the chain has reached this point, removing a link is not the proper remedy."
tommyo is correct. that is generally the rule of thumb when it comes to replacing your chain. just place your motorcycle in its center stand then gently pull it away from the back sprocket between your thumb and pointing finger. if it pulls away by half the roller as he says or halfway on the tooth of the sprocket then please do replace it. otherwise you'll just risking your precious bike to damages or worse, end up in an accident. always replace it as a SET. do not replace it one by one, replace the whole thing - chain and sprockets so that they always run in synch. also, you'll notice if the sprockets need replacement is if the teeth are already so pointed/sharp that it looks like a fillet knife rather than a triangular shape with a little bit flat surface. always keep it lubricated. lubrication is very very good, lol.
I realize this question is old, but despite good intentions, none of the answers provided are correct. Simply measuring slack in the chain (between or at the sprocket) is not sufficient as chain tension can be adjusted easily on these bikes. You need to measure how much the chain can stretch when tight.
According to Suzuki's service manual, you should put the bike on the center-stand, loosen the rear axle nut, then turn the chain adjustment nuts (on both sides) until the chain is tight. You then count 21 pins and measure the distance between the first and last one. If the distance is over 319.4 mm, the chain should be replaced.