Park Tool CC-3.2 Chain Wear Indicator
A worn chain shifts poorly and wears sprockets at an accelerated rate. The CC-3.2 is a “go-no go” gauge designed to accurately indicate when a chain reaches .5% and .75%, the points at which most chain manufacturers suggest replacement. The updated CC-3.2 is longer, more accurate and features permanent measurement markings.
Park makes a dial gauge chain wear indicator that can get bent and indicate more chain wear than is actually present. It's impossible to bend this gauge in a way that would do that. It'll save you a lot of money on cassette tooth wear over time. Worth the investment.
This tool is really handy for indicating when it is time to change your chain especially if you have more than one bike and don't keep a check on the mileage. For such a low price it can save you the added expense of replacing cog sets where the chain was not replaced soon enough.