The simplest answer is if they start having jams on the last round, or fail to lock the slide back. This is when the spring is fully extended and at its weakest. There are some pro-active methods to changing springs so you never have a failure from spring weakening. The easiest way is to take a new spring and use it as a size gauge against springs that are in use and you will notice the used springs will shrink over time. When they lose one or two whole coils in height, swap them out for new springs. Many pistols will still shoot fine with springs that have lost 2 coils in height, and some pistols 3 coils in height.