Liftpump testing on cummins isx

Anyone know how to test the performance of the electric lift pump on an isx? I see the test port on the high pressure pump, but kind of think that just tests the high pressure pump. I do not have the esn right now, I apologize. Truck rolled into my work yard this morning and I cannot get it to misbehave even though the owner said he barely made it there. I had heard that it died on him on the road a couple of weeks ago so I figured I would hear from him. Apparently he changed the filters and had a hard time getting it to prime. But then it ran fine for a week. What I found, pulled a 227 code. But I think that is a stored code not active, because there us not a check engine light. Iirc that code is high voltage in the 5 volt vref circuit. Found the secondary fuel filter a bit loose. I could spin it by hand almost half a turn. That could be the whole problem right there. But I cannot open road test it (let that cdl lapse about 15 years ago). So I just want to check some basics tomorrow. He is supposed to drop off new filters tomorrow so I plan on dissecting the primary and making sure that the fuel pick up is clear. I did check all of the fuel line connections and everything thing was tight, single tank so it can't be the dual tank leveler. And, this truck had a minor fire last year. All the fuel suction lines were replaced and most of the return lines as well. So I doubt it is a disintegrating fuel line. One thing, the owner is a bit of a phone diva. The entire time I was checking basics he was on the phone with people. At one point (and I didn't even register at the time) he told someone on the phone that the fuel returning to the tank is always dark. You may not be able to tell, but I wrote that last paragraph a few minutes ago. And I find it disturbing. Why would fuel returning to tank be dark? Disintegrating fuel line? Soot from a combustion chamber? That last might indicate a failed injector sleeve/cup. I don't know. I may be over thinking it. Any ideas are appreciated.