How to know when to change a chain?
You can measure the stretch of a chain with a chain gauge (or just a ruler) 


Alternately - the chain links are 1 inch long, so measure 12 of them with a ruler, if they are more than 1/8inch longer than this then they are worn.

You need to do this before it wears the rear cogs - it will cause the teeth to wear into sharp pointed spikes = need a new cassette.