In case you haven't noticed, the International Residential Code is starting to require air-tightness testing of ductwork in new houses. There's a right way to do that, but most hvac contractors aren't up to speed on it.

Atlanta-based sustainable-housing non-profit Southface has a nice ten-minute video up on YouTube explaining how to use a "duct blaster" to measure air duct leakage (" Duct Leakage Testing in Homes"). It won't make you an expert, but it's a good start.