Well that takes a bit of work. You have to know your body pretty well and event hen it can be tricky. The best way is to chart about three cycles to get the hang of things. This means taking your temp every morning just as you wake up without moving. It is best to take it at the same time every morning. Also you need to pay attention to your Cervical Mucus. It changes throughout your cycle. When it is like raw egg whites and when you have quite a bit of it tends to be your most fertile time. If you notice I didnt say this is when you ovulate because this can occur over several days but sometime during that time you should ovulate. Some women will feel when they ovulate by slight cramping on either the right or left side. But not all. You can do a search and find many sites that can tell you how to chart your cycles. Also reading the book Taking Charge Of Your Fertility by Toni Weschler will help you in wonderous ways.
I found that charting was extremely difficult because I have small children and often at least one of them comes to my room a few nights a week. If you get up in the night or even waking up in the night, it can throw off the whole temperature thing. I found the best way was to use the ovulation predictor kits. My husband and I chose to use the ClearPlan Fertility monitor which was expensive but oh so worth the money. A friend of mine was able to be go to an online auction and buy hers for half the cost and it was still brand new, never been opened, sealed, and still had rebate forms, etc in it (she couldn't use the rebate though, it wasn't purchased through an authorized dealer).
I found that even though I had been charting, my ovulation was off from when I actually thought it was and I was charting EVERYTHING from cervical mucus to body temp, to what I ate, to what happened in the night, etc, etc. But...with the computer monitor....I got pregnant the first try! YEAH!!!!!!
The charting can work as long as you get at least two hours of straight sleep. I have two small children also and to top it off I have had problems with insomnia since I was a young child. Charting is how I know when I got pregnant with my second child.
You can also check the position of your cervix to help know when you ovulate.
That is why I suggested the book TCOYF.
The ovulation prediction kits are ok,they never worked right for me. They were never predicting anything for me. The charting is what worked for me.
Though for some the kits work great.