When health professionals believe someone has a personality disorder, they typically run a series of medical and psychological tests and exams. These can help rule out other problems that could be causing your symptoms, pinpoint a diagnosis and also check for any related complications. These exams and tests generally include:

• Physical exam. This may include measuring height and weight, checking vital signs, such as heart rate, blood pressure and temperature, listening to your heart and lungs, and examining your abdomen.
• Laboratory tests. These may include a complete blood count (CBC), a screening test for alcohol and drugs, and a check of your thyroid function.
• Psychological evaluation. A doctor or mental health provider talks to you about your thoughts, feelings, relationships and behaviour patterns. He or she asks about your symptoms, including when they started, how severe they are, how they affect your daily life and whether you’ve had similar episodes in the past. You’ll also discuss any thoughts you may have of suicide, self-injury or harming others.They may also ask you to complete some questionnaires covering some of these aspects.

Pinpointing which personality disorder you have
It sometimes can be difficult to determine which particular personality disorder or personality disorders you have. For one thing, some personality disorders share similar symptoms. Also, a diagnosis is often based largely on how you describe your symptoms and behaviour, along with how your doctor interprets those symptoms and observes you behaving. Because of this, it can take some time and effort to get an accurate diagnosis. Be sure to stick with it, though, so that you can get appropriate treatment designed for your particular illness and situation.

Diagnostic criteria
The symptoms and clinical features for each personality disorder are detailed in a book called the Diagnostic and Statistical Manual of Mental Disorders (DSM). This manual is published by the American Psychiatric Association and is used by mental health providers to diagnose mental illnesses and by insurance companies to reimburse for treatment.
To be diagnosed with a particular personality disorder, you must meet the criteria for that disorder listed in the DSM. Each personality disorder has its own set of diagnostic criteria.

References:
http://www.nlm.nih.gov/medlineplus/personalitydisorders.html

http://www.mayoclinic.com/print/personality-disorders/DS00562/METHOD=print&DSECTION=all