What is a definitive way for a doctor to know if someone has PTSD?

Mental health professionals know about a person's mental health status and the disorders affecting them by implementing mental health assessments or psychological evaluations. Clinical psychologists are the professionals who implement complete psychological evaluations, which take several days and multiple sessions including interviews, tests, and other psychological tools to understand the person's mental and emotional processes, and personality and to identify any condition affecting their mental health. If done with competence and ethics, they are very accurate.

Regular mental assessments, on the other hand, can be implemented by many mental health professionals, but are superficial in comparison to complete psychological evaluations, which only clinical psychologists perform. But these assessments could be good enough to identify the most common mental disorders like anxiety, mood, and psychotic conditions. PTSD could be diagnosed through a mental assessment for sure.

Now, anybody could learn about a disorder and fake symptoms and stories as a way to get a mental health diagnosis, then it depends on the professional's expertise and ethics to implement a truly good process to identify how accurate/objective or not such reports/behaviors are.

While mental assessments could be easily manipulated, psychological evaluations are much harder to manipulate, and as I said before if the professional is truly competent and ethical, they would not naively rely on a verbal report but make a real evaluation of everything necessary and viable to know and understand the person's mental health/behavior.

Assessments and evaluations are not limited to conversations. Assessments, if done effectively, start with the review of health records, communication with health providers, and ideally with loved ones to ensure a good process as possible. Psychological evaluations use several tools beyond the clinical interview, including tests and questionnaires.

Again, the mental assessment does not require tests, and every practitioner/clonic has their own guidelines about them. Psychological evaluations have a multitude of clinical options and processes to be considered.