Check moving direction

[pst_mul]

I would like to use the sensors for getting the moving direction of the device and check for potential 180 turns the user takes. How can I achieve this by using sensors like the accelerometer or gyroscope?

P.S.: Using geolocation should be avoided as the exercise will mainly be indoors.

[Gondwana]

I think you'd want to use a gyroscope (orientation sensor). Beware that not all Fitbits have this sensor. You can check official store pages to find out.