How do I know if the water pump is bad
 

How do I know the water pump is bad. Are there ways that I can test to see if it is still working or not. I lose alot of coolant. Its not leaking outside and there is only a little bit of white smoke from the tailpipes. When I start it, it takes a long time to heat up but the needle just keeps on going up until it overheats. There isn't anything blocking the radiator. I know the smoke means that I probably overheated and blew a head gasket. But I don't want to replace the gasket and have it blown again cause the pump isn't working. It the chicken and egg problem.
Can anybody help me out with this.