Replacing chains regularly is the best way to ensure a long life from your drive-train. This Chain Wear Indicator will help tell you what your chain wear is, so you know when to replace it. 

Usage instructions
Place the wide end into a link on your chain and gently try push the other in. If the chain slips into the 1% gap then your chain has more than 1% wear. If the chain fits into the 0.75% gap but not the 1% gap, it has between 0.75% and 1% wear.

To get a long life out of your drive-train, your chain should be replaced once it has a wear of between 0.75% and 1.0%. If your chain is at 1% or more it should be replaced immediately, otherwise you risk massively reducing the life of your drive-train.