How to tell if water pump is bad (Merlin?)

I am having difficulties with my 1997 GMC Jimmy, specifically no heat.
I have been through the problem with rust/crap in my coolant system. Did all the cleanouts/flushes etc. So here is a bit of the history: in Nov 2002 I lost heat, the dealership flushed out my coolant system replaced the thermostat and rad cap and I had heat for about 5 months. In May of 2003 they replaced my heater core and again I had heat for a few months (but did not need it in the summer). Now its January and I took my truck in stating that heat was intermitant, they found a leak at the lower intake and replace the gasket there and replaced all coolant. After picking it up I had heat for 5 minutes and then it was gone for good.
I have read that the water pumps have been failing in my truck, I was wondering if the water pump failure could cause my no heat issue. Can anyone tell me if this is true or if there is someway to test to see if the water pump is bad?
Please help

It will be a mess but I would take off both hoses that goes into the firewall to the heater core. After you take them off some antifreeze will come out. Take a clear hose and put it on both ends of the motor side using clamps. You will be bypassing the heater core. (It would be like a U shape) Have someone start your truck and if you see the antifreeze pumps through the clear hose at a good flow then I would say the pump is working. If you see no leaks (with the clear hose) let the truck get up to normal temp. and feel the clear hose and if its hot then I would say your heater core is plugged. Thats what I did for for my maxima when I had no heat. Come to find out I took out my heater core and it was plugged. See I hate paying for labor so I would try everthing first before I took it to a garage. Good luck

your heater core is plugged you have the orange dexcool it has tendancy to turn solid when it hits air so that is not rust your getting out it is sludged up dexcool. disconnect your heater hoses take a garden hose and back flush the heater core you will see a bunch of crap come out and when you see it all cleaned out hook it up and go

Found that the hose from the overflow basin to the rad was plugged so when the engine was trying to pull fluid into the rad it could not and it caused a vacuum. The coolant was not able to flow freely through the system. Once the hose was cleaned the coolant flowed and I got heat back.
Thanks for your comments though.