In Short: Menstruation, cervical fluid, and waking temperature are the primary clues to determining fertility.
There are a number of different clues to show where a woman is in her fertility cycle, which phase of fertility she is in. Taking Charge of Your Fertility utilizes three primary ones: menstruation (menses), cervical fluid, and waking temperature (also known as basal body temperature or BBT). There is a fourth sign that some users of FAM rely on as backup information to the first three: the cervix. (TCOYF permits you to track the characteristics of the cervix, but it is informational only and is not used in any calculations.)
As an absolute minimum in the practice of fertility awareness, you only need to keep track of two of the four listed above: menstruation and temperature. Cervical fluid provides an added bonus in that it defines the onset of fertility, whereas temperature only indicates when ovulation has already taken place, perhaps too late to be of much use in achieving pregnancy. When used together, however, temperature and cervical fluid help you identify your fertile period with a high degree of accuracy.