Barbara Dehn, RN MS NP explains the different ways to go about this:
Use a basal body temperature chart. You can chart your basal body temperature. This is where you take your temperature first thing in the morning and plot it on an ovulation chart. Pre-ovulation, your temperature is low; post ovulation, your temperature is high – and ovulation occurs some time in between. If you map it out, you'll see that there's a low time and a high time… with ovulation occurring some time in between.
Use an ovulation predictor kit. You can buy an ovulation predictor kit. (Ed. Note: The test sticks look just like pregnancy tests and they usually come with 7 in a box. Some offer 20 in a box. There are also $200+ computerized kits if you really want to take this to the next level.) These tests look for an LH (luteinizing hormone) surge. 24-48 hours before ovulation, your luteinizing hormone will spike, and you can find that indicated on these urine testing kits.
Get to know your cervical mucus. Another great way is to look at your cervical mucus. Ovulatory mucus is very estrogen dependent, so it's very clear and thin… it looks like uncooked egg whites. It's glucose rich and nourishes the sperm on the way up. A woman might notice this a few days before ovulation and a few days after.
[NOTE: If you have a regular menstrual cycle, try The Cradle's Ovulation Calculator!]