There are several ways of deciding when your most fertile time is. Some women have an ovulation pain, some can detect variations in their vaginal discharge or their cervical mucus, others use LH-detecting kits or Persona kits which test the urine for the hormone that is produced immediately before ovulation.
An egg is usually released 14 days before a period begins (day 14 of a 28-day cycle, day 20 of a 34-day cycle). For women who have an irregular cycle, it can be very difficult to predict when ovulation will occur. There is also a possibility that ovulation may not occur in cycles which are less than 24 or longer than 36 days.
If you are trying to become pregnant, the question is why do you need to know your most fertile time. All it tells you is the days you need not have sex if you don’t want to. If you have sex for fun most days you are bound to hit the right time for fertility.