An accelerometer might be the only easy way, but often it's used to detect movement. It would work best if your phone is tilted forward, not flat or vertical. E

An accelerometer might be the only easy way, but often it's used to detect movement. It would work best if your phone is tilted forward, not flat or vertical. Even then, it's unlikely to be exactly aligned to your car - if you're entering the car, then it could well be perpendicular to the car. A compass will tell north, but it doesn't tell which way your head is pointed.