We intend to develop a smartphone app that can tell whether its user is a driver or a passenger in an automobile. While the core problem can be solved relatively easily with special installations in new high-end vehicles (e.g., NFC), constraints of backward compatibility makes the problem far more challenging. We design a Driver Detection System (DDS) that relies entirely on smartphone sensors, and is thereby compatible with all automobiles. Our approach harnesses smartphone sensors to recognize micro-activities in humans, that in turn discriminate between the driver and the passenger. We demonstrate an early prototype of this system on Android NexusS and Apple iPhones. Reported results show greater than 85% accuracy across 6 users in 2 different cars.