Psychological evaluation is non-invasive because at this time there is no chemical test for illness. Although many illnesses (such as depression, anxiety, and bipolar for example,) are biochemical, science hasn't figured out how to test for that yet. Also there is a lot of theory about genetics.

For personality disorders it is even more difficult to understand, such as borderline personality, which doesn't respond to meds but does respond to therapy, so it is considered more of an environmental thing. But people don't respond to the same environmental factors, so they also think there are genetic triggers.

And many things mimic each other or have overlapping symptoms. Many people also have multiple diagnosis.

So for evaluation it is primarily answering questions about feelings and behaviors, and then then the evaluator goes over the responses and has an interview with you. There are many different evaluations so it will just depend on who you are seeing.