How do you know how many DPO you are?

Saryn • My husband and I have one beautiful toddler and a pumpkin colored fur baby. Currently attempting for baby number two!
I have never taken a ovulation test, I'm assuming that would tell you the exact day you were ovulating. I suppose your body temperature would also tell you, as I'm just learning about both of these I did not test. But I do remember the day that I had really bad soreness during my ovulation week. Does the egg drop on the day that I am most sore/crampy/tender or the day after? Or maybe a few hours later?
Out of the days that I was ovulating I think that the cramping started on day five. Is that normal and should I consider the most painful day to be one DPO? Or perhaps the second day? 
I am on a competitive dodgeball team and I'm concerned about getting hit in the stomach. I am possibly around nine DPO but I'm not positive… I see a lot of people are getting positive tests for 8 DPO. I'm just wondering if I should test before my dodgeball game, maybeits too early and I'll get a false negative?? 😖👻🖖🏼