1987 Honda Civic possible water pump failure?

I recently bought this vehicle and it has been having issues with overheating consistently. I have replaced the thermostat and fan switch, both with minimal effect. My concern is that the radiator or internal coolant channels may be blocked by deposits as apparently the previous owner used water rather that antifreeze, but it occurs to me that it might also be the coolant pump. Is there any possibility that the pump might not be providing effective flow any more, and if so is there a way to check without removing the entire pump/distributor assembly? If this turns out not to be the issue, what would you suggest to help clear the internal channels?