I don't think my doctor is right

I went for my 6 month check up and blood work for my hypothyroidism with my doctor this week. I was telling her about my inconsistent periods...stopped taking birth control in July, had a period in September and October, then in January, then in June. I asked her if she thought it was caused from my thyroid and she said not likely because we are managing it with medication. She said you probably have PCOS. I have no PCOS symptoms other than irregular periods and weight gain so I'm confused. How do you test for PCOS?