If the trachea is displaced - try to determine if it has been pushed or pulled
Before deciding if the trachea is central it is important to establish that the patient is not rotated. (See the chest X-ray quality tutorial).
Trachea - Pushed or Pulled ?
If the trachea is genuinely displaced to one side, try to establish if it has been pushed or pulled by a disease process. Anything that increases pressure or volume in one hemithorax will push the trachea and mediastinum away from that side. Any disease which causes volume loss in one hemithorax will pull the trachea over towards that side.