How do you know???

Victoria • TTC#1 RAINBOW BABY=) miscarriage around 6weeks June 2015. Struggled with ovarian cancer at age 17 only have one full left ovary and a small portion of my right ovary.
How do you know the EXACT day you ovulate using an opk test? Please comment because I'm a little confused. And nobody on here will answer my question lol, they read my post but don't comment it's so frustrating!!