How to find leak in home heating duct?

How about an angled mechanic's mirror and a 90 degree flashlight?

Hmm... good idea. The shaft turns up after about three feet, so it might be hard to see with a small mirror, but I have something that might work. Access to this area in the basement is a pain (in a crawl space, three or four feet of random stuff before the ducts do anything), so I might still try to get something going with a web cam or x10 on a stick. Better to start low-tech first though.

The best way (and the way professionals do it) is to blow some "smoke" in with a high pressure blower with all the vents closed and look along the pipe for plumes of smoke.

Imagine, a "mirror" can actually bend light to my will! My god!

I was able to figure out more of the duct work but ended up really wanting to know what was going on behind a wall (the one with cold air blowing out the electric socket, adjacent to the fireplace). So, I removed the wall socket (which was hung on the drywall with some nifty no-screw brackets) and stuck a mirror and a flashlight in there. Sure enough, the disappearing duct reappeared behind the wall, in a 1' x 4' x room-height dead space that has quite a breeze blowing down it. Too bad I don't have an attic to go exporing in... my house is a contemporary design, which in my case means lots of small, unexplained dead spaces near the roof line.

Now I'm wondering if I should stuff insulation into that dead space, or if I should try to find the root-cause of the air flow. Root-cause might mean cutting into a wall, and I don't think that'll fly at my house. ("Oh, I hung that picture up there last weekend... and no, I can't move it.") Of course, I have no idea how to fill a space like that with insulation when the only access I have is the size of a wall outlet!

First of all find out if the air is blowing out or sucking in. No offense but having been in the HVAC field I have had people call with a problem of air blowing out of one place or another and shown them, by use of a match, that it was actually doing the opposite.

As for the wall outlet I would say it is just outside air being blown in due to poor insulation.

The duct work is another matter. It should be fairly easy to see if it is supply or return.Does the duct originate at the bottom of the furnace ( or for a horizontal unit, the far end from the vent pipe) all by itself or is it going to the top of the unit with all of the other ducts?If it goes to the bottom it,or the side farthest from the vent pipe, it is return. If the duct is supply then you may still be looking at the problem of outside air being blown in along the supply duct and being heated as a result. again it may be a question of poor insulation. If it is a return duct then check and see if the air is being blown in or sucked out with smoke or CAREFULLY with a match ( I would prefer if you use smoke or a light powder).

Your problem will likely still be poor insulation in anyn case. GOOD LUCK.