Have you ever charted your temperature?
You can accurately predict when you will get your period by taking your temperature at the same time every morning before you get out of bed. This thermometer should ideally be inserted vaginally, and have at least 1 decimal place, if not two.
Example:
One woman's temperature is 36.5 up until she ovulates. Then her temperature jumps to 37.2. Then the day of/before her period starts again, it dips to 36.5 again.
For women whose cycles are irregular, the time from period to ovulation varies, but the time from ovulation to period again stays the same, allowing you to accurately predict your period.
If you conceive, then the day that you would normally expect your period (based on days after ovulation), you will find your body has another spike in temperature. In my woman's example, say, 37.5.
You'll also notice other things about your body during these days.
-Maybe you have breast tenderness a day before your period.
-Maybe you're emotional the day before your period.
-Maybe you're really wet for three days before your temperature rises - your fluid has the consistency of raw egg white, and you are really horny at this time.
Conception
-If you are trying to conceive, have sex during these days prior to when your temperature increases. Some evidence suggests the last day before temperature increases is more likely to get a boy, whereas earlier more likely to get a girl since girl sperm lasts longer, but boy sperm swims faster.
If you want to avoid getting pregnant without contraception, you don't have sex until after your temperature has increased for 3 days in a row without dipping down again. After you've charted temperatures for a few months, there are extra rules you can follow pre-fertile fluid for more sex time with very low chance of getting pregnant. (check out books by Toni Weschler for more info)
You'll also notice that when you're turned on/wet, and you're not in those days leading up to ovulation, the fluid is more tacky, perhaps slightly cloudy and there's a lot less of it. It can get confusing if you've had sex recently, but the fluid mixed with semen has a distinct 'semen' smell, and is more watery instead of egg-whitey, hanging out of your vagina in one string.
Also, when you shower in the morning, you can insert a finger into your vagina and feel your cervix. If it's low, angled, and firm, you're not fertile yet. If it's high, pointing straight down and soft then you're fertile. I personally find this a small factor in measuring my fertility, and opt for just the fluid and temperature observations.