I think I'm experiencing ovulation pains. How do I know that's it for sure?

Ovulation chart. You should ovulate about midcycle. If you chart your periods, beginning with the first day of your period, you should ovulate midway to the next first day. You can also do basal body temperatures to follow your ovulation. The definitive test would be to have an ultrasound to check, but it is not necessary if you know when you ovulate.
Pains. If your pain is pretty much predictable and comes 14 days prior to your next period, it probably is safe to assume that it is ovulatory pain or what we call "mittleschmertz." if it persists for more than a few days and outside of your ovulation time, have it checked.