Some people advocate having chains in rotation, and swapping them over regularly but another way to eke the life out of your bike's drivetrain is to replace the chain before it wears.
The cheapest way is to measure it with a steel ruler; the easiest is to use a chain checker. This one is Park's cheapest.
You just hook it on to the chain, which takes about two seconds - if one end stands proud, everything's fine, if it falls into the chain, it's worn. This is two sided: one side checks 0.75% wear ('replace chain soon'), the other checks 1% wear ('replace chain immediately'). It fits all chains from 1/8in singlespeeds to 10-speeds.