it overheats. if the bearing is gone you'll hear a whine or a rattle. or it leaks water out the bottom which then follows the sump gasket around the engine which can be misleading.
yup best way (apart from the noise it'll probably be making) is to check for movement. Take it from me - if it's gone - replace it asap! We've had a metal blade come off a HZ and rip through the radiator and lodge it'self in the bonnet....