on an oldsmobile I had, the radiator went bad completely. when that happened, the water being poured in, came out of the bottom instantly.
on my 1996 mustang, radiator hose blew. but those are obvious when they break.
on my 2000 mustang, the water pump wasn't destroyed, but it was leaking, it ended up just being the water pump gasket.
If you find your waterpump pulley, look to see if the water being poured into the resivor is coming out just under that, or if it is leaking somewhere else.
on my 2000 mustang, it was fairly obvious since there was a little trail of anti freeze leaking down from the water pump and dripping onto my sway bar.
I'm not an experienced mechanic. i'm just trying to help through personal experience.