Your sense of smell and sight may be more sharp than usual. Your vagina gives off slick, stretchy discharge when you are most fertitle. Another way to tell is to take your body temperature every morning before you get out of bed, and keep a record of it. It rises a little around ovulation and stays high until your period begins. This is a method that women who are trying to get pregnant use to figure out when they are most fertile.
Menstruation is only the most obvious.
Your vaginal secretions vary depending on where you are in your cycle. You probably give off the most around ovulation, and this is slick and stretchy, like egg white. At other times it is thicker and less clear You should only be worried about a secretion, also know as discharge, if it smells really bad, if there seems to be too much of it, or if it makes you itchy or burns. In these cases, you probably have a vaginal infection, and need to see a gynecologist . Only humans, apes and some monkeys menstruate.