First you need to know where it's close to the limits, armed with a copy of your old test results will help immensely: Hydro carbons and NoX are the typical.
I'm an amateur for sure, but have spent a fair amount of time researching and tuning (2) failing cars back into local compliance (86 Samurai and 2000 MR2), and I'm in the SF Bay area, which has one of if not the most stringent set of emissions requirements.
Most shops that do emissions repairs will have an older machine handy, that's not connected to Sacramento, that helps them diagnose. A sharp technician can look up the limits for your rig, check the output on the older machine, then offer up an educated guess as to what is usually the culprep "on these old Ford 351's"...
1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift