Age is your only real guide and even that isn't always accurate. When a waterpump starts to fail you'll see coolant dripping from the weep hole at the bottom of the pump. Your best bet is to connect a small drain tube to the weep hole and direct it away from the opti. (There was a post on this or another forum detailing how to do this). This way if the pump starts to fail you won't wash out the opti.