The most rigorous way to locate duct leaks is with a blower door or similar analysis performed by a professional. This analysis measures the magnitude of your duct leakage and identifies its location. Some companies offer ductwork sealing services with a follow-up blower door check to ensure duct leakage has been reduced to acceptable levels.
A blower door is a special instrument used to measure air leakage in a building shell and its ductwork. The equipment consists of a temporary door covering which is installed in an outside doorway and a blower which forces air into or out of the building. The blower door measures how leaky the building and ductwork are, and can be used to find the location of the major leaks. Without a blower door, finding the leaks in the ductwork can be difficult since the ducts are often in hard to reach areas such as the attic or crawl space and the leaks are usually hidden from view by duct insulation.
*Source – U.S. Energy Information Administration