There are two ways to check the water pump without removing it from the engine. A defective pump must be replaced. Both a new and a restored assembly can be used as a replacement.
The water pump is equipped with inspection (or vent) holes. If the seal fails, coolant will start leaking from the holes. Get a flashlight and a small mirror, then remove the timing belt cover. Using the flashlight and mirror, locate the inspection holes on the bottom of the rear/lower side of the pump and check to see if coolant is leaking from them.
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If the pump shaft bearings fail, a howling sound may be heard at the front of the engine while it is running, which should not be confused with the whistling sound made by the drive belt when it is released. Bearing wear can be detected by the presence of play when rocking the pump pulley (remember to turn off the engine first).
Sometimes the wear of the pump does not show any external signs, so during a major overhaul of the engine it makes sense to disassemble the assembly and visually assess the condition of its internal components. Often the efficiency of the pump is reduced due to severe corrosion damage to the rotor blades.