To check your HVAC ducts for leaks, the Department of Energy (DOE) recommends doing the following:
Looks for gaps and cracks around your home. For example, inspect the area where your siding and chimney meets and all of the corners on your home's exteriors.
Inspect your home's interior.  This includes looking for gaps around electrical outlets, attic hatches, window frames and more.
Conduct a building pressurization test. This will help push air through cracks so you can detect a leak more easily.
Shine a flashlight over potential gaps at nighttime. Have a partner, friend or neighbor stand outside as you do this. If they see light coming from the gaps, you have a large crack.
For more tips on how to detect air leaks, visit the DOE website.