OBD-II UART: detect reverse/forward gear change

I have the OBD-II UART Model B (gyro/accel) + Arduino Uno. Hardware and sample sketches work great. I'll keep the Arduino in the car plugged in at all times. I'd like to detect via OBD the typical sequence of events when one backs out of a parking spot: 1) start the engine, 2) throw in reverse, back out clear out of the parking spot, 3) shift the gear into "D" and drive away. I can detect engine start easily with PID_RUNTIME, but I'm wondering what's the "cleanest" way to detect the reverse/forward gear shift. I could use the accelerometer to detect a short movement backward, followed by forward movement. Or I could track the RPM: a short increase just after engine start (when backing out), then a drop to zero, then a constant increase when driving away. Are there any better options? Thanks.