Probably the easiest thing to do is pull all the injectors and take them to a diesel repair shop for testing. They can check the pressures, look for leaks and examine the spray pattern. They may need to know the make, model and year of the tractor and fuel injector pump.
As I understand it some injectors can be rebuilt but some can not. Availability of parts and labor prices may dictate it easier to just buy new injectors. I think that you can expect to pay about $100 for an injector.
For do it at home, you could buy one injector and replace #1. Then if that didn't fix the problem, move the old #1 to #2 and test again. If by the time you get all of them replaced, you should have eliminated the injectors as the problem.
If your IP pump has its own crankcase and lube sump, then the only way for diesel to get to the engine is through the front seal where it is connected to the timing gears. On my IP with separate crank lube, the lift pump was actuated by a plunger that ran off of a cam on the IP crank. This plunger was sealed by an o-ring, which in my opinion is a bad seal to use in a sliding operation. It should have been a lip seal of some sort. I couldn't keep seals in it no matter what material they were made from.
On this IP there was a fill port on the top with a mushroom shaped breather cap. There was a drain plug on the bottom and part way up the side there is a fitting with a 90 degree elbow that is the overflow tube. The crankcase of the IP holds about 4 ounces of oil. When the diesel leaked past the o-ring it filled up the sump of the IP. The problem was that it came from the factory with a rubber cap on the fitting so I didn't know it was over full. It wasn't until I changed the oil that I realized I got a lot more out than 4 ounces. After that, I fixed up an overflow bottle with a hose going to the fitting. I could monitor how much diesel was getting into the sump and how much it was diluting the oil.
Eventually I got tired of fighting with the o-rings, so I removed the lift pump and made a plate and gasket to cover the hole. I had an electric fuel pump on the shelf for another project so installed that and wired it to the on side of the key switch. That solved my over flowing sump problem for a long time.
Now, I am seeing some diesel in the sump oil again, it has to be the IP pump leaking internally as there is not other place for fuel to come from. The pump has 1000 hours on it and I guess I am not too surprised that it is starting to leak a little. For now I just keep an eye on it and change the sump oil at every engine oil change.
You are getting an exceptionally high amount of fuel into the engine so it must be a fairly serious leak. I would try to find a way to rig up an overflow bottle to the IP sump, even if it means putting a hose on fill port. The idea is to leave the sump open to atmospheric pressure so that any fuel that leaks into the sump can not pressurize it. This will also allow you to monitor the sump level easily.
If the sump is not pressurized the it is not likely to be able to push fuel past the input shaft seal at the timing gear. Doing this will let you see if the fuel is coming from the pump or internally, which would mean an injector, or maybe all of them.
Let me know if I was as clear as mud. Writing is harder than thinking...
