How do I know if the lift pump is bad? Will the truck run/miss/rattle make a clunking noise etc?? Also, what are signs of the injection pump going out?
Before doing anything else, install a mechanical fuel-pressure gauge, between the fuel-filter and injection-pump; even better, use two, one where already suggested, and a second one just ahead of the lift-pump.
With a reasonable clean fuel-filter, and a factory-type diaphragm lift-pump, the rotary injection-pump, like on the 6.9/7.3, should be able to draw enough fuel through a completely dead lift-pump to run the engine.
The trouble is, running in this manner, the injection-pump will be starved of vast amount of cooling/lubricating fuel that cycles through the return-lines.
Running with a dead lift-pump has been the demise of many injection-pumps.
Then, when the injection-pump quits, someone will replace it, while not giving a thought to the lift-pump, and the cycle goes on.
Without a gauge, you can remove the fuel line, ahead of the filter, and have someone crank over the engine, while you aim the fuel-line into a jug.
If the pump is good, it should throw a strong gusher of fuel.
A healthy diaphragm pump only holds about 7 PSI.