It may be diagnosed by a general practitioner and then you will be seen by a dermatologist and a rheumatologist. There is no specific test to diagnose psoriasis but in some cases a biopsy may be done to exclude any other issues. There is no diagnostic tool for psoriatic arthritis but if you have back, joint and or tendon pain and a history of psoriasis in the family it is a strong indicator of psoriatic arthritis.