How do I know if a holistic doctors is really good or just a snake oil salesman?

Check credentials. Check credentials and reviews on the many sites that do this. Ask around, especially any other doctors or nurses you may know.
Check them out. Google can provide a lot of information. Reputation and character also help answer that. Were you referred by a fiend? Results speak for themselves.....Interview the doctor. Rapport and instinct also help define that for you, and look at credentials. Ask questions. "snake oil salesmen" tend to have poor results and patients figure it out quickly.