Most women ovulate kind of right in between their last period and their next period, around day 14-16 of a 28 day cycle. Another clue is discharge that is clear and stringy, like egg whites.

However, that's not set in stone, and many women don't have a perfect 28 day cycle to begin with. It's tough to know exactly when you are ovulating. That is why birth control needs to be used every time, not just when you "should" be ovulating, because when you "should" be ovulating isn't always when you do. Some women even get pregnant if they have sex during their period.