Parkinson's disease is usually diagnosed by a neurologist who can evaluate symptoms and their severity. There is no test that can clearly identify the disease. Sometimes people with suspected Parkinson's disease are given anti-Parkinson's drugs to see if they respond. Other tests, such as brain scans, can help doctors decide if a patient has true Parkinson's disease or some other disorder that resembles it. Microscopic brain structures called Lewy bodies, which can be seen only during an autopsy, are regarded as a hallmark of classical Parkinson's. Autopsies have uncovered Lewy bodies in a surprising number of older persons without diagnosed Parkinson's -- 8% of people over 50, almost 13% of people over 70, and almost 16% of those over 80, according to one study. As a result, some experts believe Parkinson's disease is something of an "iceberg; phenomenon," lurking undetected in as many as 20 people for each known Parkinson's patient. A few researchers contend that almost everyone would develop Parkinson's eventually if they lived long enough.