To see how much wear a chain has, use a ruler marked in inches. Bicycle chains use a half-inch pitch, so when you line one rivet up at the beginning of the ruler, there will be (with a new chain) a rivet at each inch marking on the ruler (each second rivet on the chain).
As a chain wears, it gets slightly longer, due to looseness in each connection in the chain. A chain has "stretched" 1% in length when the rivet at 11 inches moves to be at 11 and 1/8 inches. (To be precise, that's 1.13% stretch).
Using this guide, you can see how much "life" a chain still has in it (or how long ago you should have replaced it!).