You don't! Thus, if you continue having unprotected sex, or if she does not take the "pill," then she COULD get pregnant. Even if she did NOT have PCOD, the "rhythm method" (which I guess you are doing) is NOT a real good birth control method!

LH predictor kit!

There are ovulation kits available. Clomid helps to define ovulation.

Please use a urine ovulation kit and she should start checking daily from day 10 from 1st day of pregnancy and if test positive have sex same day and next day for next chance. If not ovulation in 2-3 months of periods please see a doctor.
Good luck!

Change in cervical mucus, consistent temperature, pelvic pain, breast tenderness.

You wouldn’t know when she is ovulating since periods are irregular with PCOS. She still needs use effective contraception like birth control pills. Your girlfriend needs to see her OBGyn and discuss the best contraception for her.
Take care and stay healthy!

This is a very difficult question. She has to keep track of her cycles. If they are not regular, then she needs to see an OB/gyn. There are medications to aid with achieving pregnancy by making her ovulate.