LT1 Water Pump Test?

All, I have a 94 LT1 in my truck. I just bought a new huge custom aluminum radiator, shroud, and SPAL fan (yeah, I live in 112 degree Arizona). I also bought a new water pump and thermostat(180) in the process. Well, after changing everything, it behaves strangely. At idle, the fan seems to keep it at 180 degrees. However, when you drive it, the temperature shoots up immediately out of control. It I tested the thermostat, my last test before calling the radiator shop is to make sure the water pump is working. With it getting hot soooo fast, it almost seems like what would happen if the water pump just stopped pumping. Yes, when I put it back together, I DID put the coupler back on the splines on the block. I even replaced the o-ring seals in the process. I made sure (with the picture in my shop manual) that the coupler went on with the groove facing the engine side. So, it HAS to be pumping right? Is there an easy test to see if the pump is working as it should? It's kind of hard with the reverse flow cooling system to just leave the cap off and watch when the flow starts! How much coolant should be moved at idle speed when the thermostat is open? Is it a slow trickle at that engine speed? Help is appreciated. I feel bad having cooling problems after spending so much money on the radiator and parts. I'm not sure what to do at this point. I wanted to check with you guys before I called the radiator shop back. Let me know if you have any water pump test suggestions. TIA Scott