A lot of it's based on your symptoms and health history, but it could take some time to figure it out. And part of the process is ruling out other conditions that look like Parkinson's. That's one reason it's important to go to a doctor who knows a lot about it. Early on, it's easy to miss.
If you do have it, your doctor might use the Hoehn and Yahr scale to tell you what stage of the disease you're in. It ranks how severe your symptoms are from 1 to 5, where 5 is the most serious.