Keeping an eye on chain wear is only one half of the preventative maintenance game plan when it comes to drivetrains. Cassettes and chainrings can wear as well, and the Unior 1658 Sprocket Wear Indicator can help you gauge when it's time to toss that cassette in favor of a new one.
The tool consists of a chrome-plated steel bar attached to a length of chain. Drape the chain over a cog, making sure the tip of the steel bar is also resting in the valley between two teeth. Also ensure the very last link of the chain is not in contact with the cog. Apply a 10Nm force to the end of the bar and try to press the last link down into the tooth valley. If it goes in easily, the cassette can be considered to be in good shape. If it doesn't, it's time for a new cog set.