The doctors can only go by what you tell them and what symptoms you have. I filled out a questionaire that they gave me too. Those things help the doctor determine if you have BPD or OCD or any other mental ilness.
There is no approved lab test or anything for these disorders yet. Maybe in the future, but not yet. So yes, as mentioned, they will look at symptoms and your behaviors and see if you meet certain diagnostic criteria.
Kristina