Unfortunately, there isn't such a passive method to diagnosis of mental disorders. This is because mental health/ disorders are multifactorial and consists of biological, psychological, and environmental factors, and as such, isn't as straightforward as observing something under a microscope. So, diagnosing someone requires extensive background information (usually through interviews) and administration of psychological tests/ screening tools. With a variety of different information retrieved, the clinician will have better knowledge as to the diagnosis that can be made.

Actually, this is quite similar to the work of a medical professional. They too will engage in the process of differential diagnosis (e.g., determining which is the correct diagnosis out of a variety of diagnoses). They might order additional tests so that more information can be generated to make the right diagnosis.

For example, a complaint of a headache could be due to a variety of reasons. The doctor will need to rule out the most obvious reasons first, and if not due to those reasons, will then go to reasons which are more rare/ less likely. That is when more extensive tests may be ordered. A clinical psychologist or psychiatrist works in the same way, but for mental disorders.

Hope that helps!