I was at work when I posted the above--I'm back home now and found the form the emissions testers are supposed to ask for: Colorado Dept of Revenue, Division of Motor Vehicles, Emissions Section, form
DR2365, "Vehicle Evaluation". Only place I know to get it is at the Colorado Dept of Public Health and Environment Emission Technical Center at 2450 W. 2nd Ave in Denver. You'll need to make an appointment. They don't do the emissions test there, just the inspection.
The emissions testers I know of in the area are:
S&S Services, Inc., 11705 Teller Street, Suite F, Broomfield. I've usually used this guy. Super nice, professional, and good prices.
Lee Myles Auto Care, 6270 Arapahoe #1, Boulder. I used these guys once when S&S wasn't available. Higher price and they curiously didn't ask for the DR2365. They're supposed to.
Al's Front Range Diesel Emissions, 1115 S Bowen Cir, Longmont. Never used this guy. Price is between the other two, but he was booking about 6 weeks out when I called.
All of this is assuming your address for registering the truck is in the Colorado Motor Vehicle Emission Testing Area, which is shown on this map:
If you're out of that area, no worries, no DR2365, no emissions test. Easy peasy.