If all else fails:
Run the motor in both directions and observe brush sparking. Set the
brushes to minimize sparking in the preferred direction.
This assumes that the
brushes can be positioned for optimum timing. However in many small dc motors, the brushes are not adjustable, so neither rotation direction is favored.
Regarding compressor rotation direction: If there is a belt drive, the tension will be on the slack side of the belt. Also often, there will be a rotation arrow somewhere on the casing. Good luck.