10 hilariously insightful foreign words. A smog check is a test done on the exhaust system of a motor vehicle to determine how many pollutants, and of which type, the vehicle is emitting.It also includes an assessment of other parts of the vehicle to make sure that they are in working order, especially those parts which pertain to emissions control. A smog check is part of a state's vehicle inspection that tests the types and amounts of gases released in the exhaust of a car or truck.