wide band a/f gauge.. pretty much the only way to do it with certainty..

does the testing station rev that engine when you test, or do they do it at idle? If you're leaning out at idle, it could be a fuel pressure or injector issue.. doubtful though..

what kind of mileage do you get? what does your temperature gauge read most often?

 

at the dmv here we idle through temp when warmed up stays right around mid range today I was gettin 12 city 14 to 16 highway thats rollin on 35" nitto mud grapplers

 

sounds about right/normal..

if you don't have an exhaust leak hiding out somewhere (which is what I'm figuring it has to be), you may want to double check your IAT and o2 Sensor.. their pretty much whats left that can lean you out.. If it were running THAT lean though, it would certainly run hot(ish) too..

I'd get a can of seafoam and run it through the booster.. I'd then watch real close like around the manifolds/headers, all the way to the tips to see if there was any smoke coming out where there shouldn't be smoke coming out- like, anywhere other than the tips..

your spark plugs can tell you if you've been running rich or lean, but only after a good while of being either.. w/o a wideband a/f gauge, I'm at a loss as to offer any more help.