If you remember when the first day of your last period was, then you can work out approximately when you will ovulate. I usually number the days in my diary, so I know approximately what days I will be most fertile. If you know this, then you know when to start looking for changes in your mucus - although, I have found this to be difficult as certain things (like residual sperm from BD) can confuse it. So I usually just use my calendar as a guide. If you want more accurate info, you could buy an ovulation predictor kit from the chemist, which will tell you when you are ovulating for sure.