How do you know EXACTLY when you ovulated??

Natalie • Pregnant with baby # 2!

So I see so many women on here say they are 8 dpo or 9 dpo.....and I’m wondering how you know exactly what day you ovulated?? We do the OPKs but once I find my surge I just know 24-36 hours I should ovulate?? So not a definite just a guesstimate.