How do you know when you ovulate

I am sorry if this is such a basic question but it seems that some women seem to know precisely when they ovulate? In my past 2 pregnancies, I guessed the ovulation time though the positive indicator on an OPK kit would would not know when I actually ovulate/egg released. Apart from going for an u/s, how can you tell for sure?

for me it is just charting temps and knowing when i should expect o so that i can pay attention to my body.  i've never used a monitor - so i don't know about that.  but for alot of women, there are o cramps along with the increased mucus and really soft cervix.  good luck!

cm and cervix after +opk

info in faq under o+12