Finding air leaks in ducts
Google "Smoke Pencil" or "Wizard Stick". Both use fog machine technology to vaporize a small amount of glycerine, producing a cool stream of smoke. The former is the professional product; the latter is a cheaper toy, but probably good enough for your purposes. (And, you definitely don't want to be messing around with flames in a dense tangle of air ducts. Could get exciting fast.)