PTSD (post traumatic stress disorder) is diagnosed by a psychiatrist based on a clinical interview of the patient. An inventory of symptoms is gathered, and a flowchart of how the different anxiety disorders are diagnosed is illustrated here:
So if there is a trauma history and flashbacks (recurring images of the trauma), then this may alert the psychiatrist that a person may have PTSD. The rest of the clinical interview involves gathering more history of the trauma and a complete inventory of anxiety symptoms. In PTSD, there are symptoms of recall (flashbacks, nightmares), avoidance (avoid any reminders of the trauma), and hypervigilance (jumpy, easily startled).
