This is hard to answer, are you meaning how do you know you are pregnant, or how do you know it is the perfect time of the month? When you look at a calander and keep track of your period, you will have an answer as to when it is perfect time for your egg to get fertalized. http://www.drmirkin.com/women/8786.html This web site will explain the timing of your egg. If you're asking about if you are pregnant or not, best to wait at least a month if not more before taking a test. However, some believe you can find out within 48 hours after sex.