You would be surprised to know how many times we get emails with people asking 'How can I tell if my motorcycle's chain and sprockets need replacing?'. And we can understand the question because if you search the Internet for answers you don't know what's right and what's wrong as so many different answers are to be found.
The thing is is modern chains and sprockets keep getting better but they're still extremely important items as they do a very tough job.
As chains wear they stretch and each link becomes slightly stretched, making the chain longer over time. When they are brand new they stretch a noticeable amount but as they bed in you can rely on them and check them less (once every three or four weeks is fine).
There are a couple of ways of judging if you motorcycle chain needs replacing; if you can lift it off the sprocket or if you've run out of adjustment. If you can lift one link more than one or two millimeters off the rear sprocket it need replacing. In the past the test was if you could lift a link far enough to see light between it and the sprocket, then the chain was past its best but modern O and X ring chains stretch less than older motorcycle chains and if yours is this bad it needs replacing urgently. The other way of telling is if you've ran out of space on the adjusters at the rear spindle. Some unscrupulous types used to take a link out of a worn chain to try and hide the fact it's bad but it's not so common these days. If you're in doubt a good supplier should be able to tell you how many links your motorcycle's chain should have so you can count them and check.
A visual check's very important too and that may require a good clean first with chain cleaner or paraffin and a small brush. Any damage you can spot is serious cause for concern. Look for side plate condition and missing rollers as simple stone jamming in the wrong place can cause serious problems. Check the far side of the chain for rust too, especially near the pins, because people tend to lube this side less. Another thing to look out for are tight spots which can build up in chains, often caused by debris such as rust or grit working in between the links and plates. It's something to look out for from new as well and certainly something you should bear in mind when checking chain tension (always spin the wheel and check chains along their whole length).
The easiest way is to measure the vertical free play in the lower run of the chain at various points during its rotation. It's less common with modern )-ring and X-ring chains but even these can develop tight links. Sometimes cleaning or using a thin oily lube such as Scottoiler oil can free them off but it often means the chain needs replacing.
The sprockets are important and need a good check too. Not only can the teeth wear and become more hook-like, but if chain alignment's out they can wear more one one side than the other. So look for uneven pattern to the teeth edges.
It's always best to replace the chain and sprockets as a set rather than doing one without the other which can cause premature wear to the new part. When choosing replacement parts 'cheap once' are generally false economy so use good suppliers. Their experience not only helps you get a decent product for a fair price but they'll be able to advise on different brands, or potentially different sprocket sizes if you want to change gearing settings.