Do some reading on NFP (Natural Family Planning).
NFP makes use of 'basal body temperature' to track your body temp daily, in the morning before you get out of bed. Your temp rises just a tiny bit just after you ovulate. Other signs of ovulation include a difference in your internal mucus signs (you are 'wetter'), and your cervix will lower and feel less firm. When you are not fertile, your cervix will feel like the tip of your nose, when fertile, it will feel more like your lower lip.
For the most part, it takes quite a few months of cycles to learn your signs of fertility.