An air flow testing device for use with a clothes dryer to indicate an obstruction in the exhaust system. Three potential states of exhausted air are monitored by the testing device when the dryer's lint screen is removed from its opening. In the first state, when the exhaust system is unobstructed, an impeller of the device freely turns within a casing held against the lint screen opening as the dryer is operating. In the second state, when the exhaust system is blocked, the same impeller remains static and does not rotate. In the third state when the exhaust system is partially blocked, the impeller moves and may stop. If either a fully or partially blocked state is detected, a user can simply clear the system including the tubing and flapper to resolve this common dryer problem without calling in a repair person.