Not quite sure what you mean, but: Is the water pump malfunctioning? The car is overheating? Well, if it is the water pump, you would need to remove it to check how well it is functioning.
You can't really check the water pump except that with the belt off, you wiggle it to look for play in the bearing. When the bearing gets loose, it causes seal failure and massive coolant loss. But after 100k miles it is time to do the pump anyway. Not that hard or expensive. Do the thermostat as well, just in case. But if no coolant loss, then overheating is probably from something else, like bad plug wire, intake manifold leak, plugged catalytic converter, retarded ignition, head gasket, etc. If the radiator was not clear, then the electric fan would go on often. Not likely radiator. Depends also on when heats up. If city driving, then likely to be fan or switch. If highway driving, then plugged convertor more likely. Put cooling system pressure tester on radiator cap. Check for leaks under higher than normal pressure. See if holds pressure. Then start engine and see if cylinder leak causes oscillation.
Get the radiator rodded or replaced. Next time, please post under Chevrolet or GMC. This is a VW only section.
