Personalization and automation in future smart vehicles hinge on accurately identifying the driver and passengers in the vehicle. Traditional approaches either require additional infrastructure or impose assumptions about how users interact with their smartphones. The recent proliferation of commercial smartwatches enables new opportunities to solve this problem due to the fixed position of the watch on the wrist. We use this observation to motivate WatchUDrive, our smartwatch-based application for identifying whether the wearer is the driver or a passenger in a vehicle. We evaluate two smartwatch sensing modalities for driver vs. passenger differentiation: the accelerometer and the camera. Using 40 in-vehicle episodes collected from 8 users and 8 different vehicles, we show that the accelerometer yields 90% accuracy within 10 seconds, whereas the camera only yields 62% accuracy within 110 seconds.