how can i know ?

There are a few options to know when your most fertile time is, and you can read more about them at the Fertility and your Menstrual Cycle page.

The basic method (that you will need to do, regardless of other methods used) is to keep track of your menstrual cycles each month, and learn how long your cycle is. ("Day 1" is the first day of your period, then count how many days until your next period, which again, would start over with "Day 1").

Once you know your menstrual cycle, if it is fairly regular, then you can estimate when you ovulate (this is your most fertile time). Ovulation occurs in the middle of your cycle, but is difficult to pinpoint with just the calendar method alone.

Other, more accurate, measures to estimate ovulation include taking your basal temperature every morning, and noticing the trend when your temperature increases slightly. There are also ovulation predictor kits, and you can track your cervical mucus (click on the link above, and it describes the "how to" for each method) for changes that indicate ovulation.