Greetings and Welcome to Just answer.
The best way is you put oil dye in the fuel, then take the oil filler off. Run the engine and see if there is dye coming around the drive gear under there.
No that is the only way that GM says.
I have no acccess at the moment to part numebrs of the pumps so I am not sure if they will interchange.
Did you check the return lines under the valve covers? They can crack and cause this or there may be a line leaking under the valve cover.
Did you run the engine with the valve covers off to check for leaks?
This can only be the front of the injection pump, the injectors, the high pressure lines or the return lines and seals.
Was there dye in the system for that test?
OK, if you did not see any at the injectors, lines or return lines, then all that is left is the pump.
The dye is the only way that GM has or puts instructions for on these pumps. No rebuilds.
The are seals at the back of the pump but only the front seal is inside the crank case so if the rear seal was leaking, it would dump on the ground.