Is it possible to test a water pump for proper function (ability to move a certain volume of coolant) without disconnecting it from the engine? If so, how would one do that?
I've been wondering this too as a lot of overheating related questions come up.
There are 2 ways water pumps fail - leaks and impeller failure. Leaks can typically be seen, so they are easier to diagnose. Impeller failure is difficult to detect.
Typically, the engine will overheat, but the heater core and radiator will still be cold. With a thermostat failure, the radiator will be cold, but the heater core should still get hot. This leads to a possible false positive if the heater core is plugged and the thermostat failed.