Finding blockage in HVAC sealed in drywall ceiling

I recently finished most of my basement, including a drywall ceiling. Underneath the drywall are several HVAC lines that branch off the main line. Each of those lines had a shutoff twist-thing, and I *thought* I had made sure they were all open before sealing off the ceiling.

Naturally, as soon as winter came, one of the second floor bedrooms turned out not to have heat. It's only getting a trickle of air, so I assume that I probably left the shut-off closed on that line, or that it's blocked somewhere else.

The only problem is that I don't know how to find the line. Measuring the house seems theoretically possible, but prone to error. If I hire a duct-cleaning company, can they run a camera through the ducts and tell me where any obstruction is? What should I ask for?

Thanks!

You might be able to borrow or rent a thermal imaging device. Turn the thermostat way up, then follow the ducts. You should see where it cools off, indicating the blockage. If you befriend the guys at the local fire station, they may have one and use it to practice finding an internal wall fire if you ask nicely.

That's a great idea, thanks!