What is the best way to diagnose simple pneumothorax

Recent literature supports the use of bedside transthoracic ultrasound in the hands of experienced emergency medicine physicians as a superior modality for the diagnosis of pneumothorax in comparison to plain chest X-ray, particularly in the supine patient.
However if plain chest X-ray is used, a posterior-anterior (PA) film in full expiration should be used to detect a small pneumothorax.