Locate the furnace, air conditioner or combination HVAC unit.
Turn on the system at the thermostat. If you are working with a combination heating and cooling system you can test the unit in either AC or heating mode.
Allow the unit to run for 3 minutes. Place your hand near either duct. As hot air rises, for heating or combination systems the supply duct is usually the higher of the two ducts.
Feel for a difference in temperature between the two ducts. The supply duct will become hotter for heating or cooler for air conditioning whereas the temperature of the return duct will remain fairly constant.