You need to read a book, such as TCOYF.
The most definitive proof that ovulation has happened (short of ultrasound or conceiving a baby) is temping, but that becomes apparent after the fact (though you will start to see patterns.) If your goal is to avoid conceiving, you want to avoid sex or unprotected sex during the fertile period leading up to ovulation (usually 3-5 days, sometimes slightly less or slightly more.) You can tell you are in this zone by the quality of your cervical mucus, and some choose to check the feel and position of their cervix internally, and some have secondary symptoms such as ovulation pain. You can also use ovulation tests or a fertility monitor or some other device. The mucus is the main thing to look out for.
Get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me. -Muhammad Ali