Open the bonnet with the engine running and see if you notice the belt tensioner "jumping" which would indicate the bearing is failing. You can replace the pulley on the tensioner without replacing the whole tensioner; the only time you might have to replace all is if the spring in the tensioner has failed. Be mindful that the bolt to the pully on the tensioner is most likely left-hand threads. The reason we mention the tensioner is that it might be possible the belt is not driving the water pump properly to circulate the water at the correct speed. However, you would probably also notice the volt meter (should be amp meters but that is another rant) needle either jumping or indicating a lower voltage output that normal.
Pump failures are actually bearing failures. It is very rare that the impellers in the pump fail. The only thing would be that the bearing is seizing and thus the pump is not turning; however, that would eventually wear the drive belt; inspect the drive belt to see if it looks as if it was spinning on a pulley that was not rotating. You should also be able to visually see if the pump pulley is seizing.
Thanks Kimberly, but the fan got replaced a while back and works great. I've also considered the tensioner, and although it's in the "normal" range, it's on the low side. I'll watch it to see if it "jumps". The belt is turning the water pump pulley. This is an inline 6, and not transversely mounted.
Old test for water pumps was to squeeze the upper radiator hose and release it to see if you feel a surge of water move through r the system.
