Hello, I don't know how to tell if or when coolant is flowing; but, I hope you are certain changing the water pump is necessary... I've drained out old coolant, flushed the cooling system with cleaner, installed a new thermostat & new radiator cap and filled the system with 50/50 premixed antifreeze... The tubes inside the radiator cleaned up & opened up pretty good by flushing the system with flushing cleaner before installing the antifreeze... I top off the system until the overflow/expansion tank on the left inner fender well is half full...

My dashboard has a needle gauge for engine temp (ignition ON I've never seen a red light for temp) My temp gauge needle never got past 1/4 scale for several years, then suddenly it runs normal about 3/8 scale or a little more... Fellows here, said it was a poor electrical connection on the coolant temp sensor (I actually don't know where that sensor is threaded into the head (I think it is near the thermostat) <--- a pair of twisted wires go to a sensor in that area on my truck... My cooling fan is oem clutch type also... I don't think my truck overheats X fingers...

So, my truck began having lack of power then hard to start when HOT and now will not start at all... My mechanic friend says the carburetor is flooding the engine... We recently changed the carburetor... My truck has a good strong spark at the plugs NGK BPR5ES-11... I pulled one plug and it was totally black, too much fuel... Fellows here have said PLUGGED VENTS in the fuel bowl or AIR LEAKS at the carb to intake manifold or STICKING/open/closed NEEDLE SEAT are most likely the problem... FSM & YouTube also have info that a dirty EGR valve can cause a no start problem... My vacuum lines and all else checks out good... Maybe it's a fuel carburetor problem on your truck as well..?

So hopefully any day now I me we whoever will begin removing the oem Mikuni carburetor to try cleaning & install a kit of partial parts into it & adjust the float, etc... Wished I owned an ultrsonic cleaner BUT I wish even more that you make sure it's really necessary for you to change your water pump before doing so...

One of us is gonna win these battles
Aloha --
George