You asked how to tell if it's not working. You only have moments to see the temp gauge rising very fast, before your engine is seriously damaged. If the impeller is cracked and near self-destruction, there will be no external sign of it whatsoever. That's why people are replacing them proactively. Mine had some very small cracks at about 53,000 miles.
Were you just kidding about what a water pump is? It moves water from the engine, where it's heated, through the radiator, where it's cooled, and back to the engine. One of the things you can not go without except for a very short time.
